docs: def download(self, bucket, key, fileobj, transfer_config=None, subscribers=None):
          ''' download a file using Aspera '''
          check_io_access(os.path.dirname(fileobj), os.W_OK)
          return self._queue_task(bucket, [FilePair(key, fileobj)], transfer_config,
                                  subscribers, enumAsperaDirection.RECEIVE)