on can read the contents of
        `filename` and optional `revision`.

        For example::

          my_db = MongoClient().test
          fs = GridFSBucket(my_db)
          grid_out = fs.open_download_stream_by_name("test_file")
          contents = grid_out.read()

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

        Raises :exc:`~gridfs.errors.NoFile` if no such version of
        that file exists.

        Raises :exc:`~ValueError` filename is not a string.

        :Parameters:
          - `filename`: The name of the file to read from.
          - `revision` (optional): Which revision (documents with the same
            filename and different uploadDate) of the file to retrieve.
            Defaults to -1 (the most recent revision).
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`

        :Note: Revision numbers are defined as follows:

          - 0 = the original stored file
          - 1 = the first revision
          - 2 = the second revision
          - etc...
          - -2 = the second most recent revision
          - -1 = the most recent revision

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