It won't be able to stop the upload, it would just stop the update or insert after the upload. You would have to use client validation to stop the upload in the first place.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.