public class MimeHelper
    {
        public static int MimeSampleSize = 256;
        public static string DefaultMimeType = "application/octet-stream";
        [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 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;
            }
        }
    }