Re-enable ‘open file’ button.

Re-enable ‘open file’ button.

Originally disabled as a way to not violate the ‘inline script/style’ not allowed permission. We resolve the inline styling issues by moving styling to CSS. 

Also only accept PDF documents in the file dialog (yes it’s a weak form of filtering since not every OS/browser enforces it, but when it works it'll help improve the UX). As far as I know, this doesn't work on Windows but works on OSX.