docs: def file_url(self, entity_id, filename, channel=None):
          '''Generate a URL for a file in an archive without requesting it.
  
          @param entity_id The ID of the entity to look up.
          @param filename The name of the file in the archive.
          @param channel Optional channel name.
          '''
          url = '{}/{}/archive/{}'.format(self.url, _get_path(entity_id),
                                          filename)
          return _add_channel(url, channel)