Bemuse uses a special type of file called BMS, or Beat Music Sequence. This isn't a type of file (there's no .bms file), rather, a type of folder. Keep that in mind.
How do you get them? BMS folders aren't easy to come along. However, I've found a few sources.
Once you download these, unzip them then drag one song's folder into the page! I've previously had issues with audio not working, but that shouldn't be an issue for others.