Hey, we have one for videos, why not have one for text files? I will assume that they're made for the default SoundFont unless told otherwise.
To share on PC: When loading your file under Manage Attachments, go through the following folders: mariopaintcomposerpc -> Mario Paint Composer PC -> Prefs -> Song name]MarioPaint.txt. Select the txt file.
To share on Mac: (This is a guess, because I don't have MPC on my Mac) Same as with PC, but the folders are in the Applications library, and it's called mariopaintcomposerosx and Mario Paint Composer OSX.
To listen: Right click on the attachment name, select Save Target As (or whatever your browser says), then save it with the default name in your Prefs folder, then load up MPC, and it should be on the list of saved songs.
To share SoundFonts: Simply provide a download link. To download, click the link, choose to save it, and save it with the name soundset.sf2 (for 1.0) or soundset3.sf2 (for 2.0)

I can't share any at the moment, because MPC is on my other computer.

EDIT: I have a song.

Mario Circuit from Super Mario Kart. No loop, default SF.

1 attached file

Super Mario Kart - Mario Circuit]MarioPaint.txt