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)