New file extensions

[tomangelo]

1. I'm for new file extensions as long as it's possible to keep backward compatibility with .txt files. File structure won't change much (if at all), so only difference is file extension, which only make difference on Windows OS. It would be bigger issue if we would want to change internal file structure.
2. I think long extensions will be best. Easier to remember than shorter extensions ("Was it .sac like SAtCom, or .sat like SATcom, or .sam like SAtcoM, or [...]?"). It shouldn't be a problem for current OSs. It also makes it conflict-proof (.cbot and .satcom aren't used, .scene isn't really popular, used by few 3D editors).