The entry point for this flow mixin - Schedules new file transfer.

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()