Don't know. I don't think the prototype stuff is too complex. You're asking the thingummy to do a lot (open 'open dialog', attach file, upload file, report when successful).
Actually I don't want to upload the file to the server, I want the client code to process it.
prototype is designed to be os independent. jquery the same. You'll need to use ajax I think to get hold of files.
I think you'll need an invisible file field anyway to open the old dialog. Add a button or clickable image or whatever to act as a pseudo file field (linked via js to file field). See the link I gave you for the source code. In fact, it seems that this script does what you want as the actual uploading has been disabled. Result!