Guys the same thing happened to me. Me and tech support are tight after all the tickets I've sent 'em. Try the following:
>DISable your popup blocker. It shouldn't be en
>Never click "back" to get to the common app page to load the essay
>Close your browser (i use firefox), reboot your computer, then try again
>Make sure the files are under 500kb
>If all this hasn't worked, tell tech support, and they'll have you send them the files in an email, with info on which college you're sending it to, what essay choice topic it is, your username, what version of your app it's going on, and whatever else they ask for, and they'll upload it for you
Stop coming down on tech support. They're working hard around the clock for everyone who was too lazy to get started on apps earlier.