Only reload assets in supported formats.
Enable sound reloading.

That was easy! I wonder if we want to restart existing playbacks with
the updated sound. I can see cases for and against, and it's a little
complicated so I'll leave it for now.
Have AssetPack.getFile return a File, which is reloadable.

This breaks any code that called getFile, but it's an easy fix. Just add
toString() to read the File as a string.