From Mario Fan Games Galaxy Wiki
Type Audio- MIDI
Mimetype audio/midi (*.mid, *.midi)
application/x-midi (*.mid)
Module formats also count as MIDI-type formats.

MIDI, or Musical Instrument Digital Interface, is a musical format that sends song data directly to the sound card to be synthesized and relayed back to the user. MIDI used to be commonly used in fangames thanks to its extremely small file size compared to WAVE, MP3 and Ogg Vorbis sound formats. In addition to small file sizes, MIDIs typically load very quickly. However, MIDI sounds tend to be highly computerized and unrealistic. While some game consoles (such as the Nintendo DS) have different sound chips to make up for this, personal computers still remain sounding highly computerized. In fact, MIDI playback often sounds worse on newer Windows PCs than it did on Windows 98-era machines.

An alternative method to getting a higher quality sound has been replacing the General MIDI sound bank with other higher quality sound banks and canvases, though this method is specific to a user's sound card hardware.

VGMusic is the leading source of MIDI music for fangaming.

Due to the low-quality music MIDI usually produces, some MFGGers have begun using the Ogg Vorbis sound format or even SPC for music in their fangames. The latest version of Game Maker, GameMaker: Studio, doesn't even support MIDIs directly. However, MIDIs can be converted to WAW, MP3 or Ogg Vorbis with using a soundfont.