Mod format. music files. we all know about em, and some of us even revive our old game tunes from time to time to get that little pang of joy from the Amiga golden days... The format seems all but dead- and now everything is mp3, ogg, mmf etc...
MP3 is much more flexible than MOD. Think it this way: MP3 "supports" (plays) just not only MODs but SID/AHX/XM/whatever formats...
Edit: in fact when considering file size MOD is not the best option. Even a dummy song wastes few kilobytes for nothing when a simple SID tune could be few hundreds bytes and not bigger than 64kB. AHX is another small format, AHX tunes are never larger than 64kB excluding comments.