'Primarily used for the S3ObjectStore - get the status of data transfer
if the file is not in cache'

@web.expose
@web.json
def transfer_status(self, trans, dataset_id, filename=None):

    data = self._check_dataset(trans, dataset_id)
    if isinstance(data, string_types):
        return data
    log.debug(('Checking    transfer    status    for    dataset    %s...' % data.dataset.id))
    if data.dataset.external_filename:
        return True
    else:
        return trans.app.object_store.file_ready(data.dataset)
