[Mac] NightlyBuild is not identified to the OS as able to open MIDI files
Using: Nightly Build e02be8b (Aug 29, 2013) on OSX 10.8.4
The Resource Fork info (I think they're still using that term in OSX for it) for the MuseScore app does not include MIDI as an input format.
If you right-click (option-click) on a MIDI file in OSX and go to the "Open With" menu, among the applications that can open it, MuseScore 1.3 will be listed, but not NightlyBuild. Contrast that with a MSCZ file where both MuseScore and NightlyBuild will be listed, and NightlyBuild is the default.