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