Theoretically, up to 16MB can be uploaded as an attachment to a post. In practice, people have reported trouble with uploads over 10MB or so. If you need somewhere else to upload the file, you might be able to put it in your OneDrive folder (if you have Windows 8 or Windows 10) and get a share link somehow, though I wouldn't be able to tell you how - I personally use DropBox (which is a very similar software, regardless of your OS you can get 2GB of space for free and it's definitely possible to share a link). Anything of the kind will work well. In the future it'll be possible to upload them to the NeoLemmix website, but I've decided to hold off on implementing this until the new formats version of NeoLemmix is ready for general use.
Also note that if you get a timeout error, and your file is under 16MB, try using a different browser. Firefox in particular seems to have bugs for some people when trying to upload files.