Allow PDF uploads in image upload

@stvstnfrd - tada!
I'm pretty sure this is out of scope, but it would be nice if there was an ajax call to retrieve UPLOADABLE_FILE_TYPES so that we could just have the javascript check in that list. As it is now, if you for some reason wanted a thing not a pdf or not an image to be uploadable, you'd have to edit the code in two places (again). This is I think a future feature though.