how to identify the extension/type of the file using C#?

Question

i have a workflow where user is allowed to upload any file and then that file will be read.

Now my question is if user have image file xyz.jpg and he renamed it to

Answer 1

See PInvoke.net for more detail

   [DllImport("urlmon.dll", CharSet = CharSet.Unicode, ExactSpelling = true, SetLastError = false)]
    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);

Sample usage:

  public string MimeTypeFrom(byte[] dataBytes, string mimeProposed) {
   if (dataBytes == null)
     throw new ArgumentNullException("dataBytes");
   string mimeRet = String.Empty;
   IntPtr suggestPtr = IntPtr.Zero, filePtr = IntPtr.Zero, outPtr = IntPtr.Zero;
   if (mimeProposed != null && mimeProposed.Length > 0) {
     //suggestPtr = Marshal.StringToCoTaskMemUni(mimeProposed); // for your experiments ;-)
     mimeRet = mimeProposed;
   }
   int ret = FindMimeFromData(IntPtr.Zero, null, dataBytes, dataBytes.Length, mimeProposed, 0, out outPtr, 0);
   if (ret == 0 && outPtr != IntPtr.Zero) {
    //todo: this leaks memory outPtr must be freed
     return Marshal.PtrToStringUni(outPtr);
   }
   return mimeRet;
}

// call it this way:
Trace.Write("MimeType is " + MimeTypeFrom(Encoding.ASCII.GetBytes("%PDF-"), "text/plain"));

Another example:

/// 

/// Ensures that file exists and retrieves the content type 
/// 
/// 
/// Returns for instance "images/jpeg" 
public static string getMimeFromFile(string file)
{
    IntPtr mimeout;
    if (!System.IO.File.Exists(file))
    throw new FileNotFoundException(file + " not found");

    int MaxContent = (int)new FileInfo(file).Length;
    if (MaxContent > 4096) MaxContent = 4096;
    FileStream fs = File.OpenRead(file);


    byte[] buf = new byte[MaxContent];        
    fs.Read(buf, 0, MaxContent);
    fs.Close();
    int result = FindMimeFromData(IntPtr.Zero, file, buf, MaxContent, null, 0, out mimeout, 0);

    if (result != 0)
    throw Marshal.GetExceptionForHR(result);
    string mime = Marshal.PtrToStringUni(mimeout);
    Marshal.FreeCoTaskMem(mimeout);
    return mime;
}