code: def _format_download_uri_for_extension(etextno, extension, mirror=None):
      
      mirror = mirror or _GUTENBERG_MIRROR
      root = mirror.strip().rstrip('/')
      path = _etextno_to_uri_subdirectory(etextno)
  
      uri = '{root}/{path}/{etextno}{extension}'.format(
          root=root,
          path=path,
          etextno=etextno,
          extension=extension)
  
      return uri
docs: Returns the download location on the Project Gutenberg servers for a
      given text and extension. The list of available extensions for a given
      text can be found via the formaturi metadata extractor.