One of the great things about browsers is the way you can trust them to be reasonably sandboxed from the rest of your machine. Features like this chip away at that trust model and open up new attack vectors against unsuspecting users. I get the usability benefits, but the security implications are scary. How easy would it be to trick my mother into uploading her profile picture and have the site change that file into a malicious executable without her realizing? At a minimum, I'd hope you would be restricted from changing file permissions or editing any executables.