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copied!<p>As mentioned, MIME magic is the only way to do this. Many platforms provide up-to-date and robust MIME magic files and code to do this efficiently. The only way to do this in .NET without any 3rd party code is to use <code>FindMimeFromData</code> from urlmon.dll. Here's how:</p> <pre><code>public static int MimeSampleSize = 256; public static string DefaultMimeType = "application/octet-stream"; [DllImport(@"urlmon.dll", CharSet = CharSet.Auto)] private extern static uint FindMimeFromData( uint pBC, [MarshalAs(UnmanagedType.LPStr)] string pwzUrl, [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer, uint cbSize, [MarshalAs(UnmanagedType.LPStr)] string pwzMimeProposed, uint dwMimeFlags, out uint ppwzMimeOut, uint dwReserverd ); public static string GetMimeFromBytes(byte[] data) { try { uint mimeType; FindMimeFromData(0, null, data, (uint)MimeSampleSize, null, 0, out mimeType, 0); var mimePointer = new IntPtr(mimeType); var mime = Marshal.PtrToStringUni(mimePointer); Marshal.FreeCoTaskMem(mimePointer); return mime ?? DefaultMimeType; } catch { return DefaultMimeType; } } </code></pre> <p>This uses the Internet Explorer MIME detector. This is the same code used by IE to send a MIME type along with uploaded files. You can see the <a href="http://msdn.microsoft.com/en-us/library/ms775147%28v=vs.85%29.aspx" rel="noreferrer">list of MIME types supported by urlmon.dll</a>. One thing to watch out for is <code>image/pjpeg</code> and <code>image/x-png</code> which are non-standard. In my code I replace these with <code>image/jpeg</code> and <code>image/png</code>.</p>

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