code: def _determineLength(self, fObj):
          
          try:
              seek = fObj.seek
              tell = fObj.tell
          except AttributeError:
              return UNKNOWN_LENGTH
          originalPosition = tell()
          seek(0, self._SEEK_END)
          end = tell()
          seek(originalPosition, self._SEEK_SET)
          return end - originalPosition
docs: Determine how many bytes can be read out of C{fObj} (assuming it is not
          modified from this point on).  If the determination cannot be made,
          return C{UNKNOWN_LENGTH}.