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

I tried to open a file with an extension name of adb in Finder (to be precise, PathFinder). When I clicked the file, vi editor is activated to open the adb file. I didn't do anything with the adb file, so my guess is that vi editor changed the system setup to open it whenever unknown file format is opened.

How can I change this behavior? I want to open TextMate as a default.

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

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.