The ui isn't very good. The static header looks like the button that you press to upload.
Calling it "not very good" is a lot nicer than it deserves. It makes no sense to sacrifice functionality for "beauty" in a file upload widget. (And it's not like the rest of it is especially nice looking anyway.)
I'm also really kind of wondering why it's so large. It takes up 258180 pixels by default--that's about a third of my browser window. Unless you're putting this on a page that's meant only for file uploading, it's probably going to take up too much room.
The fact that gigantic, non-functional, unwieldy things like this (and TinyMCE, which happens to have been developed by the same people) are considered the best solutions is one of the reasons I've really come to dislike working with the big open-source CMS's.