Returns True if there's a file in the current feed with the
       given file_name in the current feed.

def _HasFile(self, file_name):
    """Returns True if there's a file in the current feed with the
       given file_name in the current feed."""
    if self._zip:
      return file_name in self._zip.namelist()
    else:
      file_path = os.path.join(self._path, file_name)
      return os.path.exists(file_path) and os.path.isfile(file_path)