You quite simply cannot upload files to a website from an iPad because iOS has no user-facing concept of a file system. If you want to upload a file you will have to either use an app or use a Mac or a PC. Forget all the hype about Flash, this is probably the single biggest obstacle to the realisation of the post-PC world. Your greatest hope would be to find a 3rd party browser app that has Dropbox integrated, but even then I doubt that it would work. I will be delighted if someone can prove me wrong on this.