The usual: png, jpg, gif, pdf, doc, docx, txt, ppt, pptx, mp4, m4v, m4r, mov, wmv, avi, mpg, mpeg, swf, flv, torrent, cue, afm, amxd, maxpat, otf, sib, ptb, mid, midi, gp5, gpx, fxp, fxb, vst, mod, it, xm, mtm, nsf, ttf, sav, epub, mobi, vst3, and alp.
Most music file types are deliberately not included.
If you want to include a track as a bonus item in an album download, upload it like any other track, but check “bonus track.”