Corresponding notes in a MIDI will be made sounds in the Thirty dollar Website by GD Colon. A MIDI is a sequence of notes. You can inspect contents of a MIDI using FL Studio, which you can download for free (the trial lasts forever and doesn't restrict you too much)
You will be given a chance to switch track names once you upload, so if you have a MIDI file, you can upload it right away.
Each MIDI track will correspond to an instrument. The name of an instrument is how it's written when you save a file from the site. You can check an instrument's internal name by looking at the str attribute when using inspect element. Or you can see all the codes here.
Please name your tracks Instrument-BasePitch (e.g., mariopaint-C5 or noteblock_harp-F#5 (using 'b's as flats are fine).). The rightmost dash separates them. In FL Studio, a "MIDI out" instance is a track; you rename that instrument. The base pitch for most instruments is F#5. If an instrument sounds lower to start off with, the base pitch should be lower, to prevent it from playing too low. Likewise, if your track has notes that are too high, set the base pitch higher.
The base pitch is the default pitch you hear when you click a sound. For example, all noteblocks have the base pitch C5, because that's what I hear by default.
If a track is named perc it will be treated as percussion (see: MIDI channel 10, or FL Studio FLEX's percussion presets).
Tracks resulting in completely blank names will be omitted.
If a track name is "MIDI OUT", you should change or omit it. Avoid putting weird characters in track names, since that could result in unpredictable behavior on the site.