Change default MIDI file extension to '.mid'

[Davide Liessi]

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'.

What do you think?

Best wishes.
Davide