Allow PDF uploads in image upload

@jrbl & @stvstnfrd - Changes to allow PDF uploads using image upload feature.
@stvstnfrd - just to clarify - the scope of this project is to simply add pdfs as allowed filetypes in exactly the same way that it's currently done for images, not to improve the way it's currently done for images, which could be a separate project down the road if it felt high payoff.