Image formats supported:
Images can be imported e.g. to use as as movables/sprites for enemies, items, ..., or as wall and floor texture for your rooms, and more. In many places, they're limited to a 32x32 size.
Digital audio formats supported:
The sounds are limited to a few seconds length, which with some care also suffices for looped background ambient sounds. If you want long music tracks, use MIDI.
Music formats supported:
.midi/.MIDfiles (via this helper software)
RFS supports General MIDI 1/2 for game music. However, it does so in a reasonably modern way with an internal cross-platform software synthesizer (via this other software library), and a full, integrated instrument set. This gives you consistent and reliable playback.
If you think MIDI is outdated and bad, here is why RFS uses MIDI in more detail.
If you find a file that is not working, let the community know so it can be fixed. RIFF/RMID is currently unsupported, you need to use pure General MIDI music files.