lient().test
          fs = GridFSBucket(my_db)
          # get _id of file to read.
          file_id = fs.upload_from_stream("test_file", "data I want to store!")
          grid_out = fs.open_download_stream(file_id)
          contents = grid_out.read()

        Returns an instance of :class:`~gridfs.grid_file.GridOut`.

        Raises :exc:`~gridfs.errors.NoFile` if no file with file_id exists.

        :Parameters:
          - `file_id`: The _id of the file to be downloaded.
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`

        .. versionchanged:: 3.6
           Added ``session`` parameter.
        r3