EncoderFilter Class

public class EncoderFilter : MimeFilterBase

                    public static void Base64EncodeFile (string inputFileName, string outputFileName)
{
    using (var input = File.OpenRead (inputFileName)) {
        using (var output = File.Create (outputFileName)) {
            // A FilteredStream passes all data that is written to (or read from) it through
            // each of its filters, in the order that the filters were added.
            using (var filtered = new FilteredStream (output)) {
                filtered.Add (EncoderFilter.Create (ContentEncoding.Base64));

                input.CopyTo (filtered);

                // Flush the filtered stream so that the encoder writes out any remaining
                // buffered data to the output stream.
                filtered.Flush ();
            }
        }
    }
}