Files created by apps, or the app files themselves? For the former, it's all on you. That is, if you din't backup your data, it's gone. There is no undelete function. If you are referring to the app files themselves, they should be available through Google Play (assuming you downloaded from there in the first place). Simply go to Play > My apps > All. Any app (even paid ones) that you ever downloaded from that account will be available to re-download.