Hey guys! I was on ifile messing around with some files and found out how to fix your old or worlds that you downloaded that are corrupted! It's really simple. 1st of all, you have to close minecraft, then go to ifile, go to your minecraft worlds folder, and press your world. Now all you have to do is, click the blue arrow pointing to the right by the db folder, go down to ownership, and make sure both Owner and Group are both on mobile! And do the same with level.dat. That's it! Now open your world and enjoy.
Here's another tutorial on how to make your old worlds infinite if you want to: http://www.minecraftforum.net/forums/minecraft-pocket-edition/mcpe-mods-tools/2161797-short-tutorial-how-to-make-old-worlds-infinite