code: def open_download_stream(self, file_id):
          
          gout = GridOut(self._collection, file_id)
  
          # Raise NoFile now, instead of on first attribute access.
          gout._ensure_file()
          return gout
docs: Opens a Stream from which the application can read the contents of
          the stored file specified by file_id.
  
          For example::
  
            my_db = MongoClient().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.