docs: def StartFileFetch(self, pathspec, request_data=None):
      """The entry point for this flow mixin - Schedules new file transfer."""
      # Create an index so we can find this pathspec later.
      self.state.indexed_pathspecs.append(pathspec)
      self.state.request_data_list.append(request_data)
      self._TryToStartNextPathspec()