def METHOD_NAME(self, files_info=None):
    """Got full size of thumbnail directory.
    Args:
        files_info (List[FileInfo]): Prepared file information about
            files in thumbnail directory.
    Returns:
        int: File size of all files in thumbnail directory.
    """
    if files_info is None:
        files_info = self.get_thumbnails_dir_file_info()
    if not files_info:
        return 0
    return sum(
        file_info.size
        for file_info in files_info
    )