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.

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)