En utilisant .NET, comment pouvez-vous trouver le type MIME d'un fichier basé sur la signature du fichier et non sur l'extension

Je cherche un moyen simple d'obtenir un type MIME où l'extension de fichier est incorrecte ou non donnée, quelque chose de similaire à cette question uniquement dans .Net.

Dans Urlmon.dll, il existe une fonction appelée FindMimeFromData.

De la documentation

La détection de type MIME, ou «reniflage de données», fait référence au processus de détermination d'un type MIME approprié à partir de données binaires. Le résultat final dépend d'une combinaison d'en-têtes de type MIME fournis par le serveur, d'une extension de fichier et / ou des données elles-mêmes. Habituellement, seuls les 256 premiers octets de données sont significatifs.

Alors, lisez les premiers (jusqu'à) 256 octets du fichier et passez-le à FindMimeFromData.

J'ai finalement utilisé urlmon.dll. Je pensais qu'il y aurait un moyen plus simple mais cela fonctionne. J'inclus le code pour aider quelqu'un d'autre et me permettre de le retrouver si j'en ai besoin.

using System.Runtime.InteropServices;

...

    [DllImport(@"urlmon.dll", CharSet = CharSet.Auto)]
    private extern static System.UInt32 FindMimeFromData(
        System.UInt32 pBC,
        [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
        [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
        System.UInt32 cbSize,
        [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
        System.UInt32 dwMimeFlags,
        out System.UInt32 ppwzMimeOut,
        System.UInt32 dwReserverd
    );

    public static string getMimeFromFile(string filename)
    {
        if (!File.Exists(filename))
            throw new FileNotFoundException(filename + " not found");

        byte[] buffer = new byte[256];
        using (FileStream fs = new FileStream(filename, FileMode.Open))
        {
            if (fs.Length >= 256)
                fs.Read(buffer, 0, 256);
            else
                fs.Read(buffer, 0, (int)fs.Length);
        }
        try
        {
            System.UInt32 mimetype;
            FindMimeFromData(0, null, buffer, 256, null, 0, out mimetype, 0);
            System.IntPtr mimeTypePtr = new IntPtr(mimetype);
            string mime = Marshal.PtrToStringUni(mimeTypePtr);
            Marshal.FreeCoTaskMem(mimeTypePtr);
            return mime;
        }
        catch (Exception e)
        {
            return "unknown/unknown";
        }
    }

J'ai trouvé une solution codée en dur, j'espère que j'aiderai quelqu'un:

public static class MIMEAssistant
{
  private static readonly Dictionary<string, string> MIMETypesDictionary = new Dictionary<string, string>
  {
    {"ai", "application/postscript"},
    {"aif", "audio/x-aiff"},
    {"aifc", "audio/x-aiff"},
    {"aiff", "audio/x-aiff"},
    {"asc", "text/plain"},
    {"atom", "application/atom+xml"},
    {"au", "audio/basic"},
    {"avi", "video/x-msvideo"},
    {"bcpio", "application/x-bcpio"},
    {"bin", "application/octet-stream"},
    {"bmp", "image/bmp"},
    {"cdf", "application/x-netcdf"},
    {"cgm", "image/cgm"},
    {"class", "application/octet-stream"},
    {"cpio", "application/x-cpio"},
    {"cpt", "application/mac-compactpro"},
    {"csh", "application/x-csh"},
    {"css", "text/css"},
    {"dcr", "application/x-director"},
    {"dif", "video/x-dv"},
    {"dir", "application/x-director"},
    {"djv", "image/vnd.djvu"},
    {"djvu", "image/vnd.djvu"},
    {"dll", "application/octet-stream"},
    {"dmg", "application/octet-stream"},
    {"dms", "application/octet-stream"},
    {"doc", "application/msword"},
    {"docx","application/vnd.openxmlformats-officedocument.wordprocessingml.document"},
    {"dotx", "application/vnd.openxmlformats-officedocument.wordprocessingml.template"},
    {"docm","application/vnd.ms-word.document.macroEnabled.12"},
    {"dotm","application/vnd.ms-word.template.macroEnabled.12"},
    {"dtd", "application/xml-dtd"},
    {"dv", "video/x-dv"},
    {"dvi", "application/x-dvi"},
    {"dxr", "application/x-director"},
    {"eps", "application/postscript"},
    {"etx", "text/x-setext"},
    {"exe", "application/octet-stream"},
    {"ez", "application/andrew-inset"},
    {"gif", "image/gif"},
    {"gram", "application/srgs"},
    {"grxml", "application/srgs+xml"},
    {"gtar", "application/x-gtar"},
    {"hdf", "application/x-hdf"},
    {"hqx", "application/mac-binhex40"},
    {"htm", "text/html"},
    {"html", "text/html"},
    {"ice", "x-conference/x-cooltalk"},
    {"ico", "image/x-icon"},
    {"ics", "text/calendar"},
    {"ief", "image/ief"},
    {"ifb", "text/calendar"},
    {"iges", "model/iges"},
    {"igs", "model/iges"},
    {"jnlp", "application/x-java-jnlp-file"},
    {"jp2", "image/jp2"},
    {"jpe", "image/jpeg"},
    {"jpeg", "image/jpeg"},
    {"jpg", "image/jpeg"},
    {"js", "application/x-javascript"},
    {"kar", "audio/midi"},
    {"latex", "application/x-latex"},
    {"lha", "application/octet-stream"},
    {"lzh", "application/octet-stream"},
    {"m3u", "audio/x-mpegurl"},
    {"m4a", "audio/mp4a-latm"},
    {"m4b", "audio/mp4a-latm"},
    {"m4p", "audio/mp4a-latm"},
    {"m4u", "video/vnd.mpegurl"},
    {"m4v", "video/x-m4v"},
    {"mac", "image/x-macpaint"},
    {"man", "application/x-troff-man"},
    {"mathml", "application/mathml+xml"},
    {"me", "application/x-troff-me"},
    {"mesh", "model/mesh"},
    {"mid", "audio/midi"},
    {"midi", "audio/midi"},
    {"mif", "application/vnd.mif"},
    {"mov", "video/quicktime"},
    {"movie", "video/x-sgi-movie"},
    {"mp2", "audio/mpeg"},
    {"mp3", "audio/mpeg"},
    {"mp4", "video/mp4"},
    {"mpe", "video/mpeg"},
    {"mpeg", "video/mpeg"},
    {"mpg", "video/mpeg"},
    {"mpga", "audio/mpeg"},
    {"ms", "application/x-troff-ms"},
    {"msh", "model/mesh"},
    {"mxu", "video/vnd.mpegurl"},
    {"nc", "application/x-netcdf"},
    {"oda", "application/oda"},
    {"ogg", "application/ogg"},
    {"pbm", "image/x-portable-bitmap"},
    {"pct", "image/pict"},
    {"pdb", "chemical/x-pdb"},
    {"pdf", "application/pdf"},
    {"pgm", "image/x-portable-graymap"},
    {"pgn", "application/x-chess-pgn"},
    {"pic", "image/pict"},
    {"pict", "image/pict"},
    {"png", "image/png"}, 
    {"pnm", "image/x-portable-anymap"},
    {"pnt", "image/x-macpaint"},
    {"pntg", "image/x-macpaint"},
    {"ppm", "image/x-portable-pixmap"},
    {"ppt", "application/vnd.ms-powerpoint"},
    {"pptx","application/vnd.openxmlformats-officedocument.presentationml.presentation"},
    {"potx","application/vnd.openxmlformats-officedocument.presentationml.template"},
    {"ppsx","application/vnd.openxmlformats-officedocument.presentationml.slideshow"},
    {"ppam","application/vnd.ms-powerpoint.addin.macroEnabled.12"},
    {"pptm","application/vnd.ms-powerpoint.presentation.macroEnabled.12"},
    {"potm","application/vnd.ms-powerpoint.template.macroEnabled.12"},
    {"ppsm","application/vnd.ms-powerpoint.slideshow.macroEnabled.12"},
    {"ps", "application/postscript"},
    {"qt", "video/quicktime"},
    {"qti", "image/x-quicktime"},
    {"qtif", "image/x-quicktime"},
    {"ra", "audio/x-pn-realaudio"},
    {"ram", "audio/x-pn-realaudio"},
    {"ras", "image/x-cmu-raster"},
    {"rdf", "application/rdf+xml"},
    {"rgb", "image/x-rgb"},
    {"rm", "application/vnd.rn-realmedia"},
    {"roff", "application/x-troff"},
    {"rtf", "text/rtf"},
    {"rtx", "text/richtext"},
    {"sgm", "text/sgml"},
    {"sgml", "text/sgml"},
    {"sh", "application/x-sh"},
    {"shar", "application/x-shar"},
    {"silo", "model/mesh"},
    {"sit", "application/x-stuffit"},
    {"skd", "application/x-koan"},
    {"skm", "application/x-koan"},
    {"skp", "application/x-koan"},
    {"skt", "application/x-koan"},
    {"smi", "application/smil"},
    {"smil", "application/smil"},
    {"snd", "audio/basic"},
    {"so", "application/octet-stream"},
    {"spl", "application/x-futuresplash"},
    {"src", "application/x-wais-source"},
    {"sv4cpio", "application/x-sv4cpio"},
    {"sv4crc", "application/x-sv4crc"},
    {"svg", "image/svg+xml"},
    {"swf", "application/x-shockwave-flash"},
    {"t", "application/x-troff"},
    {"tar", "application/x-tar"},
    {"tcl", "application/x-tcl"},
    {"tex", "application/x-tex"},
    {"texi", "application/x-texinfo"},
    {"texinfo", "application/x-texinfo"},
    {"tif", "image/tiff"},
    {"tiff", "image/tiff"},
    {"tr", "application/x-troff"},
    {"tsv", "text/tab-separated-values"},
    {"txt", "text/plain"},
    {"ustar", "application/x-ustar"},
    {"vcd", "application/x-cdlink"},
    {"vrml", "model/vrml"},
    {"vxml", "application/voicexml+xml"},
    {"wav", "audio/x-wav"},
    {"wbmp", "image/vnd.wap.wbmp"},
    {"wbmxl", "application/vnd.wap.wbxml"},
    {"wml", "text/vnd.wap.wml"},
    {"wmlc", "application/vnd.wap.wmlc"},
    {"wmls", "text/vnd.wap.wmlscript"},
    {"wmlsc", "application/vnd.wap.wmlscriptc"},
    {"wrl", "model/vrml"},
    {"xbm", "image/x-xbitmap"},
    {"xht", "application/xhtml+xml"},
    {"xhtml", "application/xhtml+xml"},
    {"xls", "application/vnd.ms-excel"},                        
    {"xml", "application/xml"},
    {"xpm", "image/x-xpixmap"},
    {"xsl", "application/xml"},
    {"xlsx","application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"},
    {"xltx","application/vnd.openxmlformats-officedocument.spreadsheetml.template"},
    {"xlsm","application/vnd.ms-excel.sheet.macroEnabled.12"},
    {"xltm","application/vnd.ms-excel.template.macroEnabled.12"},
    {"xlam","application/vnd.ms-excel.addin.macroEnabled.12"},
    {"xlsb","application/vnd.ms-excel.sheet.binary.macroEnabled.12"},
    {"xslt", "application/xslt+xml"},
    {"xul", "application/vnd.mozilla.xul+xml"},
    {"xwd", "image/x-xwindowdump"},
    {"xyz", "chemical/x-xyz"},
    {"zip", "application/zip"}
  };

  public static string GetMIMEType(string fileName)
  {
    //get file extension
    string extension = Path.GetExtension(fileName).ToLowerInvariant();

    if (extension.Length > 0 && 
        MIMETypesDictionary.ContainsKey(extension.Remove(0, 1)))
    {
      return MIMETypesDictionary[extension.Remove(0, 1)];
    }
    return "unknown/unknown";
  }
}

Edit: Utilisez simplement Mime Detective

J'utilise des séquences de tableaux d'octets pour déterminer le type MIME correct d'un fichier donné. L'avantage de cela sur la simple recherche de l'extension de fichier du nom de fichier est que si un utilisateur devait renommer un fichier pour contourner certaines restrictions de téléchargement de type de fichier, l'extension de nom de fichier ne parviendrait pas à le détecter. D'un autre côté, obtenir la signature du fichier via un tableau d'octets empêchera ce comportement malicieux de se produire.

Voici un exemple en C #:

public class MimeType
{
    private static readonly byte[] BMP = { 66, 77 };
    private static readonly byte[] DOC = { 208, 207, 17, 224, 161, 177, 26, 225 };
    private static readonly byte[] EXE_DLL = { 77, 90 };
    private static readonly byte[] GIF = { 71, 73, 70, 56 };
    private static readonly byte[] ICO = { 0, 0, 1, 0 };
    private static readonly byte[] JPG = { 255, 216, 255 };
    private static readonly byte[] MP3 = { 255, 251, 48 };
    private static readonly byte[] OGG = { 79, 103, 103, 83, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0 };
    private static readonly byte[] PDF = { 37, 80, 68, 70, 45, 49, 46 };
    private static readonly byte[] PNG = { 137, 80, 78, 71, 13, 10, 26, 10, 0, 0, 0, 13, 73, 72, 68, 82 };
    private static readonly byte[] RAR = { 82, 97, 114, 33, 26, 7, 0 };
    private static readonly byte[] SWF = { 70, 87, 83 };
    private static readonly byte[] TIFF = { 73, 73, 42, 0 };
    private static readonly byte[] TORRENT = { 100, 56, 58, 97, 110, 110, 111, 117, 110, 99, 101 };
    private static readonly byte[] TTF = { 0, 1, 0, 0, 0 };
    private static readonly byte[] WAV_AVI = { 82, 73, 70, 70 };
    private static readonly byte[] WMV_WMA = { 48, 38, 178, 117, 142, 102, 207, 17, 166, 217, 0, 170, 0, 98, 206, 108 };
    private static readonly byte[] ZIP_DOCX = { 80, 75, 3, 4 };

    public static string GetMimeType(byte[] file, string fileName)
    {

        string mime = "application/octet-stream"; //DEFAULT UNKNOWN MIME TYPE

        //Ensure that the filename isn't empty or null
        if (string.IsNullOrWhiteSpace(fileName))
        {
            return mime;
        }

        //Get the file extension
        string extension = Path.GetExtension(fileName) == null
                               ? string.Empty
                               : Path.GetExtension(fileName).ToUpper();

        //Get the MIME Type
        if (file.Take(2).SequenceEqual(BMP))
        {
            mime = "image/bmp";
        }
        else if (file.Take(8).SequenceEqual(DOC))
        {
            mime = "application/msword";
        }
        else if (file.Take(2).SequenceEqual(EXE_DLL))
        {
            mime = "application/x-msdownload"; //both use same mime type
        }
        else if (file.Take(4).SequenceEqual(GIF))
        {
            mime = "image/gif";
        }
        else if (file.Take(4).SequenceEqual(ICO))
        {
            mime = "image/x-icon";
        }
        else if (file.Take(3).SequenceEqual(JPG))
        {
            mime = "image/jpeg";
        }
        else if (file.Take(3).SequenceEqual(MP3))
        {
            mime = "audio/mpeg";
        }
        else if (file.Take(14).SequenceEqual(OGG))
        {
            if (extension == ".OGX")
            {
                mime = "application/ogg";
            }
            else if (extension == ".OGA")
            {
                mime = "audio/ogg";
            }
            else
            {
                mime = "video/ogg";
            }
        }
        else if (file.Take(7).SequenceEqual(PDF))
        {
            mime = "application/pdf";
        }
        else if (file.Take(16).SequenceEqual(PNG))
        {
            mime = "image/png";
        }
        else if (file.Take(7).SequenceEqual(RAR))
        {
            mime = "application/x-rar-compressed";
        }
        else if (file.Take(3).SequenceEqual(SWF))
        {
            mime = "application/x-shockwave-flash";
        }
        else if (file.Take(4).SequenceEqual(TIFF))
        {
            mime = "image/tiff";
        }
        else if (file.Take(11).SequenceEqual(TORRENT))
        {
            mime = "application/x-bittorrent";
        }
        else if (file.Take(5).SequenceEqual(TTF))
        {
            mime = "application/x-font-ttf";
        }
        else if (file.Take(4).SequenceEqual(WAV_AVI))
        {
            mime = extension == ".AVI" ? "video/x-msvideo" : "audio/x-wav";
        }
        else if (file.Take(16).SequenceEqual(WMV_WMA))
        {
            mime = extension == ".WMA" ? "audio/x-ms-wma" : "video/x-ms-wmv";
        }
        else if (file.Take(4).SequenceEqual(ZIP_DOCX))
        {
            mime = extension == ".DOCX" ? "application/vnd.openxmlformats-officedocument.wordprocessingml.document" : "application/x-zip-compressed";
        }

        return mime;
    }


}

Remarquez que j'ai traité les types de fichiers DOCX différemment, car DOCX n'est en fait qu'un fichier ZIP. Dans ce scénario, je vérifie simplement l'extension du fichier une fois que j'ai vérifié qu'il a cette séquence. Cet exemple est loin d'être complet pour certaines personnes, mais vous pouvez facilement ajouter le vôtre.

Si vous souhaitez ajouter plus de types MIME, vous pouvez obtenir ici les séquences de tableaux d'octets de nombreux types de fichiers différents . En outre, voici une autre bonne ressource concernant les signatures de fichiers.

Ce que je fais souvent si tout le reste échoue, c'est parcourir plusieurs fichiers d'un type particulier que je recherche et rechercher un modèle dans la séquence d'octets des fichiers. En fin de compte, il s'agit toujours d'une vérification de base et ne peut pas être utilisé pour une preuve à 100% de la détermination des types de fichiers.

Si vous utilisez .NET Framework 4.5 ou supérieur, il existe désormais une méthode MimeMapping.GetMimeMapping (nom de fichier) qui retournera une chaîne avec le mappage Mime correct pour le nom de fichier transmis. Notez que cela utilise l'extension de fichier, pas les données du fichier lui-même.

La documentation est à http://msdn.microsoft.com/en-us/library/system.web.mimemapping.getmimemapping

Vous pouvez également consulter le registre.

    using System.IO;
    using Microsoft.Win32;

    string GetMimeType(FileInfo fileInfo)
    {
        string mimeType = "application/unknown";

        RegistryKey regKey = Registry.ClassesRoot.OpenSubKey(
            fileInfo.Extension.ToLower()
            );

        if(regKey != null)
        {
            object contentType = regKey.GetValue("Content Type");

            if(contentType != null)
                mimeType = contentType.ToString();
        }

        return mimeType;
    }

D'une manière ou d'une autre, vous devrez exploiter une base de données de MIME - qu'ils soient mappés à partir d'extensions ou de nombres magiques est quelque peu trivial - le registre Windows est un tel endroit. Pour une solution indépendante de la plate-forme, il faudrait cependant expédier cette base de données avec le code (ou en tant que bibliothèque autonome).

J'utilise une solution hybride:

    using System.Runtime.InteropServices;

    [DllImport (@"urlmon.dll", CharSet = CharSet.Auto)]
    private extern static System.UInt32 FindMimeFromData(
        System.UInt32 pBC, 
        [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
        [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
        System.UInt32 cbSize,
        [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
        System.UInt32 dwMimeFlags,
        out System.UInt32 ppwzMimeOut,
        System.UInt32 dwReserverd
    );

    private string GetMimeFromRegistry (string Filename)
    {
        string mime = "application/octetstream";
        string ext = System.IO.Path.GetExtension(Filename).ToLower();
        Microsoft.Win32.RegistryKey rk = Microsoft.Win32.Registry.ClassesRoot.OpenSubKey(ext);
        if (rk != null && rk.GetValue("Content Type") != null)
            mime = rk.GetValue("Content Type").ToString();
        return mime;
    }

    public string GetMimeTypeFromFileAndRegistry (string filename)
    {
        if (!File.Exists(filename))
        {
           return GetMimeFromRegistry (filename);
        }

        byte[] buffer = new byte[256];

        using (FileStream fs = new FileStream(filename, FileMode.Open))
        {
            if (fs.Length >= 256)
                fs.Read(buffer, 0, 256);
            else
                fs.Read(buffer, 0, (int)fs.Length);
        }

        try
        {            
            System.UInt32 mimetype;

            FindMimeFromData(0, null, buffer, 256, null, 0, out mimetype, 0);

            System.IntPtr mimeTypePtr = new IntPtr(mimetype);

            string mime = Marshal.PtrToStringUni(mimeTypePtr);

            Marshal.FreeCoTaskMem(mimeTypePtr);

            if (string.IsNullOrWhiteSpace (mime) || 
                mime =="text/plain" || mime == "application/octet-stream")                    
            {
                return GetMimeFromRegistry (filename);
            }

            return mime;
        }
        catch (Exception e)
        {
            return GetMimeFromRegistry (filename);
        }
    }

HeyRed.Mime.MimeGuesser.GuessMimeType de Nuget serait la solution ultime si vous souhaitez héberger votre solution ASP.NET sur des environnements non Windows.

Le mappage d'extension de fichier est très peu sûr. Si un attaquant téléchargeait des extensions invalides, un dictionnaire de mappage permettrait par exemple de distribuer les exécutables dans des fichiers .jpg. Par conséquent, utilisez toujours une bibliothèque de détection de contenu pour connaître le véritable type de contenu.

 public  static string MimeTypeFrom(byte[] dataBytes, string fileName)
 {
        var contentType = HeyRed.Mime.MimeGuesser.GuessMimeType(dataBytes);
        if (string.IsNullOrEmpty(contentType))
        {
            return HeyRed.Mime.MimeTypesMap.GetMimeType(fileName);
        }
  return contentType;

Je pense que la bonne réponse est une combinaison des réponses de Steve Morgan et de Serguei. Voilà comment Internet Explorer le fait. L'appel pinvoke ne FindMimeFromDatafonctionne que pour 26 types de mime codés en dur. En outre, il donnera des types de mime ambigus (tels que text/plainouapplication/octet-stream ) même s'il peut exister un type de mime plus spécifique et plus approprié. S'il ne parvient pas à donner un bon type MIME, vous pouvez aller dans le Registre pour un type MIME plus spécifique. Le registre du serveur pourrait avoir plus de types MIME à jour.

Reportez-vous à: http://msdn.microsoft.com/en-us/library/ms775147(VS.85).aspx

Cette classe utilise les réponses précédentes pour essayer de 3 manières différentes: codé en fonction de l'extension, FindMimeFromData API et en utilisant le registre.

using System;
using System.Collections.Generic;
using System.IO;
using System.Runtime.InteropServices;

using Microsoft.Win32;

namespace YourNamespace
{
    public static class MimeTypeParser
    {
        [DllImport(@"urlmon.dll", CharSet = CharSet.Auto)]
        private extern static System.UInt32 FindMimeFromData(
                System.UInt32 pBC,
                [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
                [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
                System.UInt32 cbSize,
                [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
                System.UInt32 dwMimeFlags,
                out System.UInt32 ppwzMimeOut,
                System.UInt32 dwReserverd
        );

        public static string GetMimeType(string sFilePath)
        {
            string sMimeType = GetMimeTypeFromList(sFilePath);

            if (String.IsNullOrEmpty(sMimeType))
            {
                sMimeType = GetMimeTypeFromFile(sFilePath);

                if (String.IsNullOrEmpty(sMimeType))
                {
                    sMimeType = GetMimeTypeFromRegistry(sFilePath);
                }
            }

            return sMimeType;
        }

        public static string GetMimeTypeFromList(string sFileNameOrPath)
        {
            string sMimeType = null;
            string sExtensionWithoutDot = Path.GetExtension(sFileNameOrPath).Substring(1).ToLower();

            if (!String.IsNullOrEmpty(sExtensionWithoutDot) && spDicMIMETypes.ContainsKey(sExtensionWithoutDot))
            {
                sMimeType = spDicMIMETypes[sExtensionWithoutDot];
            }

            return sMimeType;
        }

        public static string GetMimeTypeFromRegistry(string sFileNameOrPath)
        {
            string sMimeType = null;
            string sExtension = Path.GetExtension(sFileNameOrPath).ToLower();
            RegistryKey pKey = Registry.ClassesRoot.OpenSubKey(sExtension);

            if (pKey != null && pKey.GetValue("Content Type") != null)
            {
                sMimeType = pKey.GetValue("Content Type").ToString();
            }

            return sMimeType;
        }

        public static string GetMimeTypeFromFile(string sFilePath)
        {
            string sMimeType = null;

            if (File.Exists(sFilePath))
            {
                byte[] abytBuffer = new byte[256];

                using (FileStream pFileStream = new FileStream(sFilePath, FileMode.Open))
                {
                    if (pFileStream.Length >= 256)
                    {
                        pFileStream.Read(abytBuffer, 0, 256);
                    }
                    else
                    {
                        pFileStream.Read(abytBuffer, 0, (int)pFileStream.Length);
                    }
                }

                try
                {
                    UInt32 unMimeType;

                    FindMimeFromData(0, null, abytBuffer, 256, null, 0, out unMimeType, 0);

                    IntPtr pMimeType = new IntPtr(unMimeType);
                    string sMimeTypeFromFile = Marshal.PtrToStringUni(pMimeType);

                    Marshal.FreeCoTaskMem(pMimeType);

                    if (!String.IsNullOrEmpty(sMimeTypeFromFile) && sMimeTypeFromFile != "text/plain" && sMimeTypeFromFile != "application/octet-stream")
                    {
                        sMimeType = sMimeTypeFromFile;
                    }
                }
                catch {}
            }

            return sMimeType;
        }

        private static readonly Dictionary<string, string> spDicMIMETypes = new Dictionary<string, string>
        {
            {"ai", "application/postscript"},
            {"aif", "audio/x-aiff"},
            {"aifc", "audio/x-aiff"},
            {"aiff", "audio/x-aiff"},
            {"asc", "text/plain"},
            {"atom", "application/atom+xml"},
            {"au", "audio/basic"},
            {"avi", "video/x-msvideo"},
            {"bcpio", "application/x-bcpio"},
            {"bin", "application/octet-stream"},
            {"bmp", "image/bmp"},
            {"cdf", "application/x-netcdf"},
            {"cgm", "image/cgm"},
            {"class", "application/octet-stream"},
            {"cpio", "application/x-cpio"},
            {"cpt", "application/mac-compactpro"},
            {"csh", "application/x-csh"},
            {"css", "text/css"},
            {"dcr", "application/x-director"},
            {"dif", "video/x-dv"},
            {"dir", "application/x-director"},
            {"djv", "image/vnd.djvu"},
            {"djvu", "image/vnd.djvu"},
            {"dll", "application/octet-stream"},
            {"dmg", "application/octet-stream"},
            {"dms", "application/octet-stream"},
            {"doc", "application/msword"},
            {"docx","application/vnd.openxmlformats-officedocument.wordprocessingml.document"},
            {"dotx", "application/vnd.openxmlformats-officedocument.wordprocessingml.template"},
            {"docm","application/vnd.ms-word.document.macroEnabled.12"},
            {"dotm","application/vnd.ms-word.template.macroEnabled.12"},
            {"dtd", "application/xml-dtd"},
            {"dv", "video/x-dv"},
            {"dvi", "application/x-dvi"},
            {"dxr", "application/x-director"},
            {"eps", "application/postscript"},
            {"etx", "text/x-setext"},
            {"exe", "application/octet-stream"},
            {"ez", "application/andrew-inset"},
            {"gif", "image/gif"},
            {"gram", "application/srgs"},
            {"grxml", "application/srgs+xml"},
            {"gtar", "application/x-gtar"},
            {"hdf", "application/x-hdf"},
            {"hqx", "application/mac-binhex40"},
            {"htc", "text/x-component"},
            {"htm", "text/html"},
            {"html", "text/html"},
            {"ice", "x-conference/x-cooltalk"},
            {"ico", "image/x-icon"},
            {"ics", "text/calendar"},
            {"ief", "image/ief"},
            {"ifb", "text/calendar"},
            {"iges", "model/iges"},
            {"igs", "model/iges"},
            {"jnlp", "application/x-java-jnlp-file"},
            {"jp2", "image/jp2"},
            {"jpe", "image/jpeg"},
            {"jpeg", "image/jpeg"},
            {"jpg", "image/jpeg"},
            {"js", "application/x-javascript"},
            {"kar", "audio/midi"},
            {"latex", "application/x-latex"},
            {"lha", "application/octet-stream"},
            {"lzh", "application/octet-stream"},
            {"m3u", "audio/x-mpegurl"},
            {"m4a", "audio/mp4a-latm"},
            {"m4b", "audio/mp4a-latm"},
            {"m4p", "audio/mp4a-latm"},
            {"m4u", "video/vnd.mpegurl"},
            {"m4v", "video/x-m4v"},
            {"mac", "image/x-macpaint"},
            {"man", "application/x-troff-man"},
            {"mathml", "application/mathml+xml"},
            {"me", "application/x-troff-me"},
            {"mesh", "model/mesh"},
            {"mid", "audio/midi"},
            {"midi", "audio/midi"},
            {"mif", "application/vnd.mif"},
            {"mov", "video/quicktime"},
            {"movie", "video/x-sgi-movie"},
            {"mp2", "audio/mpeg"},
            {"mp3", "audio/mpeg"},
            {"mp4", "video/mp4"},
            {"mpe", "video/mpeg"},
            {"mpeg", "video/mpeg"},
            {"mpg", "video/mpeg"},
            {"mpga", "audio/mpeg"},
            {"ms", "application/x-troff-ms"},
            {"msh", "model/mesh"},
            {"mxu", "video/vnd.mpegurl"},
            {"nc", "application/x-netcdf"},
            {"oda", "application/oda"},
            {"ogg", "application/ogg"},
            {"pbm", "image/x-portable-bitmap"},
            {"pct", "image/pict"},
            {"pdb", "chemical/x-pdb"},
            {"pdf", "application/pdf"},
            {"pgm", "image/x-portable-graymap"},
            {"pgn", "application/x-chess-pgn"},
            {"pic", "image/pict"},
            {"pict", "image/pict"},
            {"png", "image/png"}, 
            {"pnm", "image/x-portable-anymap"},
            {"pnt", "image/x-macpaint"},
            {"pntg", "image/x-macpaint"},
            {"ppm", "image/x-portable-pixmap"},
            {"ppt", "application/vnd.ms-powerpoint"},
            {"pptx","application/vnd.openxmlformats-officedocument.presentationml.presentation"},
            {"potx","application/vnd.openxmlformats-officedocument.presentationml.template"},
            {"ppsx","application/vnd.openxmlformats-officedocument.presentationml.slideshow"},
            {"ppam","application/vnd.ms-powerpoint.addin.macroEnabled.12"},
            {"pptm","application/vnd.ms-powerpoint.presentation.macroEnabled.12"},
            {"potm","application/vnd.ms-powerpoint.template.macroEnabled.12"},
            {"ppsm","application/vnd.ms-powerpoint.slideshow.macroEnabled.12"},
            {"ps", "application/postscript"},
            {"qt", "video/quicktime"},
            {"qti", "image/x-quicktime"},
            {"qtif", "image/x-quicktime"},
            {"ra", "audio/x-pn-realaudio"},
            {"ram", "audio/x-pn-realaudio"},
            {"ras", "image/x-cmu-raster"},
            {"rdf", "application/rdf+xml"},
            {"rgb", "image/x-rgb"},
            {"rm", "application/vnd.rn-realmedia"},
            {"roff", "application/x-troff"},
            {"rtf", "text/rtf"},
            {"rtx", "text/richtext"},
            {"sgm", "text/sgml"},
            {"sgml", "text/sgml"},
            {"sh", "application/x-sh"},
            {"shar", "application/x-shar"},
            {"silo", "model/mesh"},
            {"sit", "application/x-stuffit"},
            {"skd", "application/x-koan"},
            {"skm", "application/x-koan"},
            {"skp", "application/x-koan"},
            {"skt", "application/x-koan"},
            {"smi", "application/smil"},
            {"smil", "application/smil"},
            {"snd", "audio/basic"},
            {"so", "application/octet-stream"},
            {"spl", "application/x-futuresplash"},
            {"src", "application/x-wais-source"},
            {"sv4cpio", "application/x-sv4cpio"},
            {"sv4crc", "application/x-sv4crc"},
            {"svg", "image/svg+xml"},
            {"swf", "application/x-shockwave-flash"},
            {"t", "application/x-troff"},
            {"tar", "application/x-tar"},
            {"tcl", "application/x-tcl"},
            {"tex", "application/x-tex"},
            {"texi", "application/x-texinfo"},
            {"texinfo", "application/x-texinfo"},
            {"tif", "image/tiff"},
            {"tiff", "image/tiff"},
            {"tr", "application/x-troff"},
            {"tsv", "text/tab-separated-values"},
            {"txt", "text/plain"},
            {"ustar", "application/x-ustar"},
            {"vcd", "application/x-cdlink"},
            {"vrml", "model/vrml"},
            {"vxml", "application/voicexml+xml"},
            {"wav", "audio/x-wav"},
            {"wbmp", "image/vnd.wap.wbmp"},
            {"wbmxl", "application/vnd.wap.wbxml"},
            {"wml", "text/vnd.wap.wml"},
            {"wmlc", "application/vnd.wap.wmlc"},
            {"wmls", "text/vnd.wap.wmlscript"},
            {"wmlsc", "application/vnd.wap.wmlscriptc"},
            {"wrl", "model/vrml"},
            {"xbm", "image/x-xbitmap"},
            {"xht", "application/xhtml+xml"},
            {"xhtml", "application/xhtml+xml"},
            {"xls", "application/vnd.ms-excel"},                                                
            {"xml", "application/xml"},
            {"xpm", "image/x-xpixmap"},
            {"xsl", "application/xml"},
            {"xlsx","application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"},
            {"xltx","application/vnd.openxmlformats-officedocument.spreadsheetml.template"},
            {"xlsm","application/vnd.ms-excel.sheet.macroEnabled.12"},
            {"xltm","application/vnd.ms-excel.template.macroEnabled.12"},
            {"xlam","application/vnd.ms-excel.addin.macroEnabled.12"},
            {"xlsb","application/vnd.ms-excel.sheet.binary.macroEnabled.12"},
            {"xslt", "application/xslt+xml"},
            {"xul", "application/vnd.mozilla.xul+xml"},
            {"xwd", "image/x-xwindowdump"},
            {"xyz", "chemical/x-xyz"},
            {"zip", "application/zip"}
        };
    }
}

J'ai trouvé celui-ci utile. Pour les développeurs VB.NET:

    Public Shared Function GetFromFileName(ByVal fileName As String) As String
        Return GetFromExtension(Path.GetExtension(fileName).Remove(0, 1))
    End Function

    Public Shared Function GetFromExtension(ByVal extension As String) As String
        If extension.StartsWith("."c) Then
            extension = extension.Remove(0, 1)
        End If

        If MIMETypesDictionary.ContainsKey(extension) Then
            Return MIMETypesDictionary(extension)
        End If

        Return "unknown/unknown"
    End Function

    Private Shared ReadOnly MIMETypesDictionary As New Dictionary(Of String, String)() From { _
         {"ai", "application/postscript"}, _
         {"aif", "audio/x-aiff"}, _
         {"aifc", "audio/x-aiff"}, _
         {"aiff", "audio/x-aiff"}, _
         {"asc", "text/plain"}, _
         {"atom", "application/atom+xml"}, _
         {"au", "audio/basic"}, _
         {"avi", "video/x-msvideo"}, _
         {"bcpio", "application/x-bcpio"}, _
         {"bin", "application/octet-stream"}, _
         {"bmp", "image/bmp"}, _
         {"cdf", "application/x-netcdf"}, _
         {"cgm", "image/cgm"}, _
         {"class", "application/octet-stream"}, _
         {"cpio", "application/x-cpio"}, _
         {"cpt", "application/mac-compactpro"}, _
         {"csh", "application/x-csh"}, _
         {"css", "text/css"}, _
         {"dcr", "application/x-director"}, _
         {"dif", "video/x-dv"}, _
         {"dir", "application/x-director"}, _
         {"djv", "image/vnd.djvu"}, _
         {"djvu", "image/vnd.djvu"}, _
         {"dll", "application/octet-stream"}, _
         {"dmg", "application/octet-stream"}, _
         {"dms", "application/octet-stream"}, _
         {"doc", "application/msword"}, _
         {"dtd", "application/xml-dtd"}, _
         {"dv", "video/x-dv"}, _
         {"dvi", "application/x-dvi"}, _
         {"dxr", "application/x-director"}, _
         {"eps", "application/postscript"}, _
         {"etx", "text/x-setext"}, _
         {"exe", "application/octet-stream"}, _
         {"ez", "application/andrew-inset"}, _
         {"gif", "image/gif"}, _
         {"gram", "application/srgs"}, _
         {"grxml", "application/srgs+xml"}, _
         {"gtar", "application/x-gtar"}, _
         {"hdf", "application/x-hdf"}, _
         {"hqx", "application/mac-binhex40"}, _
         {"htm", "text/html"}, _
         {"html", "text/html"}, _
         {"ice", "x-conference/x-cooltalk"}, _
         {"ico", "image/x-icon"}, _
         {"ics", "text/calendar"}, _
         {"ief", "image/ief"}, _
         {"ifb", "text/calendar"}, _
         {"iges", "model/iges"}, _
         {"igs", "model/iges"}, _
         {"jnlp", "application/x-java-jnlp-file"}, _
         {"jp2", "image/jp2"}, _
         {"jpe", "image/jpeg"}, _
         {"jpeg", "image/jpeg"}, _
         {"jpg", "image/jpeg"}, _
         {"js", "application/x-javascript"}, _
         {"kar", "audio/midi"}, _
         {"latex", "application/x-latex"}, _
         {"lha", "application/octet-stream"}, _
         {"lzh", "application/octet-stream"}, _
         {"m3u", "audio/x-mpegurl"}, _
         {"m4a", "audio/mp4a-latm"}, _
         {"m4b", "audio/mp4a-latm"}, _
         {"m4p", "audio/mp4a-latm"}, _
         {"m4u", "video/vnd.mpegurl"}, _
         {"m4v", "video/x-m4v"}, _
         {"mac", "image/x-macpaint"}, _
         {"man", "application/x-troff-man"}, _
         {"mathml", "application/mathml+xml"}, _
         {"me", "application/x-troff-me"}, _
         {"mesh", "model/mesh"}, _
         {"mid", "audio/midi"}, _
         {"midi", "audio/midi"}, _
         {"mif", "application/vnd.mif"}, _
         {"mov", "video/quicktime"}, _
         {"movie", "video/x-sgi-movie"}, _
         {"mp2", "audio/mpeg"}, _
         {"mp3", "audio/mpeg"}, _
         {"mp4", "video/mp4"}, _
         {"mpe", "video/mpeg"}, _
         {"mpeg", "video/mpeg"}, _
         {"mpg", "video/mpeg"}, _
         {"mpga", "audio/mpeg"}, _
         {"ms", "application/x-troff-ms"}, _
         {"msh", "model/mesh"}, _
         {"mxu", "video/vnd.mpegurl"}, _
         {"nc", "application/x-netcdf"}, _
         {"oda", "application/oda"}, _
         {"ogg", "application/ogg"}, _
         {"pbm", "image/x-portable-bitmap"}, _
         {"pct", "image/pict"}, _
         {"pdb", "chemical/x-pdb"}, _
         {"pdf", "application/pdf"}, _
         {"pgm", "image/x-portable-graymap"}, _
         {"pgn", "application/x-chess-pgn"}, _
         {"pic", "image/pict"}, _
         {"pict", "image/pict"}, _
         {"png", "image/png"}, _
         {"pnm", "image/x-portable-anymap"}, _
         {"pnt", "image/x-macpaint"}, _
         {"pntg", "image/x-macpaint"}, _
         {"ppm", "image/x-portable-pixmap"}, _
         {"ppt", "application/vnd.ms-powerpoint"}, _
         {"ps", "application/postscript"}, _
         {"qt", "video/quicktime"}, _
         {"qti", "image/x-quicktime"}, _
         {"qtif", "image/x-quicktime"}, _
         {"ra", "audio/x-pn-realaudio"}, _
         {"ram", "audio/x-pn-realaudio"}, _
         {"ras", "image/x-cmu-raster"}, _
         {"rdf", "application/rdf+xml"}, _
         {"rgb", "image/x-rgb"}, _
         {"rm", "application/vnd.rn-realmedia"}, _
         {"roff", "application/x-troff"}, _
         {"rtf", "text/rtf"}, _
         {"rtx", "text/richtext"}, _
         {"sgm", "text/sgml"}, _
         {"sgml", "text/sgml"}, _
         {"sh", "application/x-sh"}, _
         {"shar", "application/x-shar"}, _
         {"silo", "model/mesh"}, _
         {"sit", "application/x-stuffit"}, _
         {"skd", "application/x-koan"}, _
         {"skm", "application/x-koan"}, _
         {"skp", "application/x-koan"}, _
         {"skt", "application/x-koan"}, _
         {"smi", "application/smil"}, _
         {"smil", "application/smil"}, _
         {"snd", "audio/basic"}, _
         {"so", "application/octet-stream"}, _
         {"spl", "application/x-futuresplash"}, _
         {"src", "application/x-wais-source"}, _
         {"sv4cpio", "application/x-sv4cpio"}, _
         {"sv4crc", "application/x-sv4crc"}, _
         {"svg", "image/svg+xml"}, _
         {"swf", "application/x-shockwave-flash"}, _
         {"t", "application/x-troff"}, _
         {"tar", "application/x-tar"}, _
         {"tcl", "application/x-tcl"}, _
         {"tex", "application/x-tex"}, _
         {"texi", "application/x-texinfo"}, _
         {"texinfo", "application/x-texinfo"}, _
         {"tif", "image/tiff"}, _
         {"tiff", "image/tiff"}, _
         {"tr", "application/x-troff"}, _
         {"tsv", "text/tab-separated-values"}, _
         {"txt", "text/plain"}, _
         {"ustar", "application/x-ustar"}, _
         {"vcd", "application/x-cdlink"}, _
         {"vrml", "model/vrml"}, _
         {"vxml", "application/voicexml+xml"}, _
         {"wav", "audio/x-wav"}, _
         {"wbmp", "image/vnd.wap.wbmp"}, _
         {"wbmxl", "application/vnd.wap.wbxml"}, _
         {"wml", "text/vnd.wap.wml"}, _
         {"wmlc", "application/vnd.wap.wmlc"}, _
         {"wmls", "text/vnd.wap.wmlscript"}, _
         {"wmlsc", "application/vnd.wap.wmlscriptc"}, _
         {"wrl", "model/vrml"}, _
         {"xbm", "image/x-xbitmap"}, _
         {"xht", "application/xhtml+xml"}, _
         {"xhtml", "application/xhtml+xml"}, _
         {"xls", "application/vnd.ms-excel"}, _
         {"xml", "application/xml"}, _
         {"xpm", "image/x-xpixmap"}, _
         {"xsl", "application/xml"}, _
         {"xslt", "application/xslt+xml"}, _
         {"xul", "application/vnd.mozilla.xul+xml"}, _
         {"xwd", "image/x-xwindowdump"}, _
         {"xyz", "chemical/x-xyz"}, _
         {"zip", "application/zip"} _
        }

Je suis tombé sur le même problème et j'ai finalement opté pour ma propre saveur de la solution de Kirk Baucom, trouvée ici .

Il me semble que c'est l'occasion pour quelqu'un d'écrire un service de recherche en ligne.

Quoi qu'il en soit, j'espère que cela aide.

Si quelqu'un le voulait, il pourrait porter l'excellent module perl File :: Type vers .NET. Dans le code se trouve un ensemble de recherches de nombre magique d'en-tête de fichier pour chaque type de fichier ou correspond à une expression régulière.

Voici une bibliothèque de détection de type de fichier .NET http://filetypedetective.codeplex.com/ mais elle ne détecte qu'un petit nombre de fichiers pour le moment.

Cette réponse est une copie de la réponse de l'auteur (Richard Gourlay), mais améliorée pour résoudre les problèmes sur IIS 8 / win2012 (où la fonction entraînerait le plantage du pool d'applications), basée sur le commentaire de Rohland pointant vers http://www.pinvoke.net /default.aspx/urlmon.findmimefromdata

using System.Runtime.InteropServices;

...

public static string GetMimeFromFile(string filename)
{

    if (!File.Exists(filename))
        throw new FileNotFoundException(filename + " not found");

    const int maxContent = 256;

    var buffer = new byte[maxContent];
    using (var fs = new FileStream(filename, FileMode.Open))
    {
        if (fs.Length >= maxContent)
            fs.Read(buffer, 0, maxContent);
        else
            fs.Read(buffer, 0, (int) fs.Length);
    }

    var mimeTypePtr = IntPtr.Zero;
    try
    {
        var result = FindMimeFromData(IntPtr.Zero, null, buffer, maxContent, null, 0, out mimeTypePtr, 0);
        if (result != 0)
        {
            Marshal.FreeCoTaskMem(mimeTypePtr);
            throw Marshal.GetExceptionForHR(result);
        }

        var mime = Marshal.PtrToStringUni(mimeTypePtr);
        Marshal.FreeCoTaskMem(mimeTypePtr);
        return mime;
    }
    catch (Exception e)
    {
        if (mimeTypePtr != IntPtr.Zero)
        {
            Marshal.FreeCoTaskMem(mimeTypePtr);
        }
        return "unknown/unknown";
    }
}

[DllImport("urlmon.dll", CharSet = CharSet.Unicode, ExactSpelling = true, SetLastError = false)]
private static extern int FindMimeFromData(IntPtr pBC,
    [MarshalAs(UnmanagedType.LPWStr)] string pwzUrl,
    [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.I1, SizeParamIndex = 3)] byte[] pBuffer,
    int cbSize,
    [MarshalAs(UnmanagedType.LPWStr)] string pwzMimeProposed,
    int dwMimeFlags,
    out IntPtr ppwzMimeOut,
    int dwReserved);

@Steve Morgan et @Richard Gourlay c'est une excellente solution, merci pour cela. Un petit inconvénient est que lorsque le nombre d'octets dans un fichier est égal ou inférieur à 255, le type MIME donne parfois "application / octet-stream", ce qui est légèrement inexact pour les fichiers qui devraient produire "text / plain". J'ai mis à jour votre méthode d'origine pour tenir compte de cette situation comme suit:

Si le nombre d'octets dans le fichier est inférieur ou égal à 255 et que le type de mime déduit est "application / octet-stream", créez un nouveau tableau d'octets qui se compose des octets du fichier d'origine répétés n fois jusqu'au nombre total d'octets est> = 256. Ensuite, revérifiez le type MIME sur ce nouveau tableau d'octets.

Méthode modifiée:

Imports System.Runtime.InteropServices

<DllImport("urlmon.dll", CharSet:=CharSet.Auto)> _
Private Shared Function FindMimeFromData(pBC As System.UInt32, <MarshalAs(UnmanagedType.LPStr)> pwzUrl As System.String, <MarshalAs(UnmanagedType.LPArray)> pBuffer As Byte(), cbSize As System.UInt32, <MarshalAs(UnmanagedType.LPStr)> pwzMimeProposed As System.String, dwMimeFlags As System.UInt32, _
ByRef ppwzMimeOut As System.UInt32, dwReserverd As System.UInt32) As System.UInt32
End Function
Private Function GetMimeType(ByVal f As FileInfo) As String
    'See http://stackoverflow.com/questions/58510/using-net-how-can-you-find-the-mime-type-of-a-file-based-on-the-file-signature
    Dim returnValue As String = ""
    Dim fileStream As FileStream = Nothing
    Dim fileStreamLength As Long = 0
    Dim fileStreamIsLessThanBByteSize As Boolean = False

    Const byteSize As Integer = 255
    Const bbyteSize As Integer = byteSize + 1

    Const ambiguousMimeType As String = "application/octet-stream"
    Const unknownMimeType As String = "unknown/unknown"

    Dim buffer As Byte() = New Byte(byteSize) {}
    Dim fnGetMimeTypeValue As New Func(Of Byte(), Integer, String)(
        Function(_buffer As Byte(), _bbyteSize As Integer) As String
            Dim _returnValue As String = ""
            Dim mimeType As UInt32 = 0
            FindMimeFromData(0, Nothing, _buffer, _bbyteSize, Nothing, 0, mimeType, 0)
            Dim mimeTypePtr As IntPtr = New IntPtr(mimeType)
            _returnValue = Marshal.PtrToStringUni(mimeTypePtr)
            Marshal.FreeCoTaskMem(mimeTypePtr)
            Return _returnValue
        End Function)

    If (f.Exists()) Then
        Try
            fileStream = New FileStream(f.FullName(), FileMode.Open, FileAccess.Read, FileShare.ReadWrite)
            fileStreamLength = fileStream.Length()

            If (fileStreamLength >= bbyteSize) Then
                fileStream.Read(buffer, 0, bbyteSize)
            Else
                fileStreamIsLessThanBByteSize = True
                fileStream.Read(buffer, 0, CInt(fileStreamLength))
            End If

            returnValue = fnGetMimeTypeValue(buffer, bbyteSize)

            If (returnValue.Equals(ambiguousMimeType, StringComparison.OrdinalIgnoreCase) AndAlso fileStreamIsLessThanBByteSize AndAlso fileStreamLength > 0) Then
                'Duplicate the stream content until the stream length is >= bbyteSize to get a more deterministic mime type analysis.
                Dim currentBuffer As Byte() = buffer.Take(fileStreamLength).ToArray()
                Dim repeatCount As Integer = Math.Floor((bbyteSize / fileStreamLength) + 1)
                Dim bBufferList As List(Of Byte) = New List(Of Byte)
                While (repeatCount > 0)
                    bBufferList.AddRange(currentBuffer)
                    repeatCount -= 1
                End While
                Dim bbuffer As Byte() = bBufferList.Take(bbyteSize).ToArray()
                returnValue = fnGetMimeTypeValue(bbuffer, bbyteSize)
            End If
        Catch ex As Exception
            returnValue = unknownMimeType
        Finally
            If (fileStream IsNot Nothing) Then fileStream.Close()
        End Try
    End If
    Return returnValue
End Function