def default(thumbnailer, prepared_options, source_filename,
            thumbnail_extension, **kwargs):
    """
    Easy-thumbnails' default name processor.

    For example: ``source.jpg.100x100_q80_crop_upscale.jpg``
    """
    filename_parts = [source_filename]
    if ('%(opts)s' in thumbnailer.thumbnail_basedir or
            '%(opts)s' in thumbnailer.thumbnail_subdir):
        if thumbnail_extension != os.path.splitext(source_filename)[1][1:]:
            filename_parts.append(thumbnail_extension)
    else:
        filename_parts += ['_'.join(prepared_options), thumbnail_extension]
    return '.'.join(filename_parts)