When creating a new topic and the user is not allowed to attach a file, then there is indeed no button to do that.
However, if you want to reply on an already existing topic, you will see the button to choose a file and attach it. When saving the posting, it will say that it was successful, but the file is not there.
This should not be possible?
Another thing is that after having choosen a file, you will see the button choose file twice:
When creating a personal message, the user is successful in attaching a file, even when not allowed to.
Sorry, I placed this topic in the wrong category. Shouldn't be in the features request. You can move it to the correct one please.