Dear all,
many programs that can read MIDI accept both '.mid' and '.midi' as file
name extension, however many other programs only accept '.mid'.
The only noteworthy reference I could find online is
http://www.midi.org/aboutmidi/tut_midifiles.php
where they explicitly say that «the PC file extension for an SMF [Standard
MIDI File] is .MID» and they never mention '.midi'.
I think that LilyPond should use the most widely accepted '.mid' as
default extension for the MIDI files it produces, instead of current
'.midi'.