C# Signature:

    [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);

VB Signature:

Public Declare Function FindMimeFromData Lib "urlmon.dll" ( _

        ByVal pbc As Long, _
        ByVal pwzUrl As String, _
        pBuffer As Any, _
        cbSize As Long, _
        ByVal pwzMimeProposed As String, _
        dwMimeFlags As Long, _
        ppwzMimeOut As Long, _
        dwReserved As Long) As Long

User-Defined Types:

None.

Notes:

VB6 Users also use these declares

Public Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (hpvDest As Any, hpvSource As Any, ByVal cbCopy As Long)

Public Declare Function lstrlenW Lib "kernel32.dll" (ByVal lpString As Long) As Long

and this additional code

    'Convert a LPWSTR pointer to a VB string
    Private Function PtrToString(lpwString As Long) As String
        Dim Buffer() As Byte
        Dim nLen As Long

        If lpwString Then
        nLen = lstrlenW(lpwString) * 2
        If nLen Then
            ReDim Buffer(0 To (nLen - 1)) As Byte
                CopyMemory Buffer(0), ByVal lpwString, nLen
            PtrToString = Buffer
        End If
        End If
    End Function

Tips & Tricks:

• Use Marshal.StringToCoTaskMemUni() to prepare input parameters (IntPtr) declared to receive strings.
• Use Marshal.PtrToStringUni() to get the managed string back from the out parameter (IntPtr).

Sample Code:

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"));

Alternative Managed API:

Do you know one? Please contribute it!

    /// <summary>
    /// Ensures that file exists and retrieves the content type
    /// </summary>
    /// <param name="file"></param>
    /// <returns>Returns for instance "images/jpeg" </returns>
    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;
    }
    //rename crystal.jpg to .gif to test functionality!
    string getImg = Environment.GetEnvironmentVariable("windir") + "\\Web\\WallPaper\\Crystal.gif";
        string mime = getMimeFromFile(getImg);

VB6 Sample Code:

  Dim result As Long
  Dim bufferOut As String

  bufferOut = Space$(256)
  Dim buffAddr As Long
  buffAddr = StrPtr(bufferOut)

  result = FindMimeFromData(0, "file://c:/test/form1.frm", vbNullString, 0, vbNullString, 0, buffAddr, 0)
  Debug.Print PtrToString(buffAddr)