Midi audio files (.midi or .mid) are universally playable -- This statement needs to be modified, as .mid and .midi files do not work any longer on Mac OS Yosemite.
Note from LCantoni: As of Jan. 6, 2015, I don't find any official confirmation that midi files are definitively not supported under Yosemite. On the contrary, one of the new features of Yosemite is the ability to transfer midi data over Bluetooth. I do find some Mac users reporting that certain midi devices no longer worked after they upgraded to Yosemite. It appears, however, that this was due to the particular devices not having updated drivers, and not to a lack of support for midi under Yosemite. That being the case, I believe that it is still accurate to state that midi files are universally playable. But let's keep an eye on the situation.