One of the things Mac apps can do is register with the system that they can handle certain document types. Photoshop can open PSDs, PNG, GIFS, JPEGS, and so on, and it tells the system this. This way, when I right click on an image and open the Open With… submenu, Photoshop’s name is listed.
But Renoise doesn’t do this with MODs, XMs, ITs, and so on.
I wish it did. It’s just a nice little touch.