There is this post that RateU made for me - its a form wizard using template conditionals.
You would have to combine that with a field on the edit post that hides the url or direct download depending on which is selected.
You will have to make a prefix_newthread and prefix_editpost_first templates.
But I did get something working sort of. I have asked around mybbhacks, and got a little help. I was told I'd need a plug in that checks for $mybb->input and validates it. But I've never made a plug in before and don't even know where to start. Any chance that you can help me with this when ever you have the time?
I see what you want - you want that people cannot upload a file twice or not upload a file at all. Are your upload areas moderated?
I have NEARLY finished a simple XTHreads set up that will allow some people to upload and approve own upload and send it to the live downloads area. The rest are moderated.
That might help as you want them to do one or the other - right?