Recuperare estensione file da byte array

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
    );

// leggi nel buffer i primi 256 byte del file

            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);

abstract class Expander
{
    private const int ZIP_LEAD_BYTES = 0x04034b50;
    private const ushort GZIP_LEAD_BYTES = 0x8b1f;

    public abstract MemoryStream Expand(Stream stream); 

    internal static bool IsPkZipCompressedData(byte[] data)
    {
        Debug.Assert(data != null && data.Length >= 4);
        // if the first 4 bytes of the array are the ZIP signature then it is compressed data
        return (BitConverter.ToInt32(data, 0) == ZIP_LEAD_BYTES);
    }

    internal static bool IsGZipCompressedData(byte[] data)
    {
        Debug.Assert(data != null && data.Length >= 2);
        // if the first 2 bytes of the array are theG ZIP signature then it is compressed data;
        return (BitConverter.ToUInt16(data, 0) == GZIP_LEAD_BYTES);
    }

    public static bool IsCompressedData(byte[] data)
    {
        return IsPkZipCompressedData(data) || IsGZipCompressedData(data);
    }

    public static Expander GetExpander(Stream stream)
    {
        Debug.Assert(stream != null);
        Debug.Assert(stream.CanSeek);
        stream.Seek(0, 0);

        try
        {
            byte[] bytes = new byte[4];

            stream.Read(bytes, 0, 4);

            if (IsGZipCompressedData(bytes))
                return new GZipExpander();

            if (IsPkZipCompressedData(bytes))
                return new ZipExpander();

            return new NullExpander();
        }
        finally
        {
            stream.Seek(0, 0);  // set the stream back to the begining
        }
    }
}