Apple - How to set up a default program opening an unknown file extension?

Finder Directions:
Click a file of that type. Press cmd-i. Reveal the Open With: tab of the Get Info window that appears. Select the app you want. Click the Change All button below that menu.

Now, all files of the type you opened will open with the application you selected.

Check this answer which points you to RCDefaultApp so you can toy with extensions and default apps.