code: public long writeTo(OutputStream out) throws IOException {
      long size=0;
      // Only do something if this part contains a file
      if (fileName != null) {
        // Write it out
        size = write( out );
      }
      return size;
    }
docs: Write this file part to the given output stream. If this part doesn't
  contain a file this method does nothing.
  
  @return number of bytes written.
  @exception IOException	if an input or output exception has occurred.