I'm not sure there's a need for AJAX here, or that it's the best approach. You want to ping a request back to the server saying "you're about to receive this file, better make sure there's no file name conflicts, and..." then what? What's the server's response to the client going to be? You're not expecting the client side to be told to change the file name, are you? I don't think there is a client/server interaction here. Am I missing some part of the process here?
All I'd do is to upload the file to a staging area, using the MAKEUNIQUE attribute of <cffile> to make sure there are no filename conflicts, then work out what the new file name needs to be for the file respository it's going to end up in, then move it to the correct dir with the correct file name and make the matching DB insert/update/whatever. And in the ensuing response to the client, advise if the file name needed to be changed (should they need to know that).
Thank you for your suggestion, but, after spending the night working with Ben Forta's CF8, vol 2, Chap 34, working with <cfajaxproxy>, I was able to do exactly what I wanted and it appears, after some testing, to work exactly as I had envisoned it should.
Thanks again for your suggestion. I appreciate your taking the time to do so.