Inactive Recognized Developer
OK, first of all, you already asked this in another thread. DON'T DO THAT.
Without knowing what the app in question is - which I don't really expect you to tell me - I can't much help you. I assume this is a blackbox assessment, or you'd have checked the source code. If the app is managed code, you can try decompiling it (you can try this if it's native code too - IDA Pro supports ARM - but that's obviously more difficult).
The extension .DATA doesn't mean anything. There are only a few common database formats used on WP8, so it might be one of those... but then, it could also be encrypted so even if you figure out the format you'll need the key too. It could just be a flat text file that has been compressed and/or encrypted, for that matter. You could try checking the file for magic numbers, either manually using a hex editor or by checking the file using libmagic (on *nix systems, /bin/file is usually a front-end to libmagic and should be able to tell you if the file is, for example, a known compressed format).