Make the file uploading screen less obnoxious about extensions

Instead of looking for a fixed list of extensions it would be nice if it would just let us upload any file that isn't too large. Or, failing that, it would be nice if the list of approved extensions was at least a bit more exhaustive.

In particular, the list should include the .bmp extension, as this is the format of the saved game thumbnails, and saved game thumbnails are, in turn, the simplest way of taking screenshots of the game.