I think Kumar is right about that. Once the upload has started, there isn't any place to click "Cancel". Of course it's always possible for the user to abort the upload by some other brute force method, like for example closing the browser window or maybe clicking the Back button. However I too don't know what happens at that point. Perhaps you could do some experiments and see for yourself.
Or if you absolutely have to have a Cancel button, perhaps a file-upload
applet would be a solution.