code: public ByteSource getByteSource()
      {
          ByteSource bs = byteSource;
          if ( progressListener != null ) {
              bs = new ProgressByteSource( bs, progressListener );
          }
          return bs;
      }
docs: If no progress listener has been set, return the byte source set in constructor, otherwise decorate it for
  progress.
  
  @return the byte source to be used for upload operation.