Allow PDF uploads in image upload

@jrbl & @stvstnfrd - Changes to allow PDF uploads using image upload feature.
To answer my own question, we do have file upload related tests in
`oa_file_upload.js` and `oa_response.js`.

@caesar2164: Can you fix the failing test (the expected text needs to be
updated to match your new error message) and also add a test that
confirms that PDFs can be uploaded?