'Returns all filenames that exist for the given key.'

@classmethod
def _get_all_files(cls, data_usage_key):

    directory = _directory_name(data_usage_key)
    (_, filenames) = _bs_model_storage().listdir(directory)
    return [_create_path(directory, filename) for filename in filenames if (filename and (not filename.startswith('.')))]
