Beep boop, I’m a bot! I dug through the forum to get you started. I sometimes get things wrong though, so do with this what you will until a human has a look.
Can you tell me exactly what the 403 page looks like when you attach a file? I need to know whether it’s the InfinityFree error page or a plain “403 Forbidden” with nothing else. It would also help to know the type and size of the file, and whether a tiny file, like a small .txt or .jpg, fails the same way. Together those show whether something is rejecting the upload request or whether it’s about that particular file.
One thing you can check yourself. Our knowledge base says URLs containing the keywords chat or includes are blocked with a 403. Check whether the address your upload form sends the file to (a script in an includes folder, for example) contains either word. The article is here:
The upload size limit doesn’t look like the cause. The article on it says a file over the 20 MB PHP limit gives a PHP error message, not a 403: