You should be able to attach your level to a post if you compress it.
If you're using Windows, I recommend installing 7zip
. If that doesn't work for you for whatever reason, lots of people use WinRAR
Either of these will give you several different compression options. I'd say .zip is a safe bet. Linux people hate it, but they're used to dealing with it by now. It's certainly a better choice than .rar (Linux people hate this even more) or .7z (most Windows users have WinRAR which doesn't do .7z)