Add more video extensions to simfiles

Right now, the only extension allowed for a simfile's video is AVI. Back in the 3.9 days this was fine as pretty much everyone used AVI XviD then (I don't remember seeing MPEG at all, but I'm pretty sure 3.9 was still compatible with it). Now SM5 supports many more codecs such as MP4 and MKV and containers including H264/265 and Theora. While uploading a video with a different extension works, ZIv will change the extension to AVI. As long as the SM and SSC have the extension for the video changed, it will work in SM5. However, I have no idea what happens in 3.9. It might ignore the video or even crash (the former of which is fine because people should be switching to SM5 anyway, but I want them to at least be able to play the song).

So TL;DR add more extensions so I can upload videos without the fear of making people's games crash.