docs: def read_envvar_file(name, extension):
      """
      Read values from a file provided as a environment variable
      ``NAME_CONFIG_FILE``.
  
      :param name: environment variable prefix to look for (without the
          ``_CONFIG_FILE``)
      :param extension: *(unused)*
      :return: a `.Configuration`, possibly `.NotConfigured`
      """
      envvar_file = environ.get('{}_config_file'.format(name).upper())
      if envvar_file:
          # envvar set, load value as file
          return loadf(envvar_file)
      else:
          # envvar not set, return an empty source
          return NotConfigured