Some mathjax distributions come with the unpacked versions removed as
per the upstream instructions for optimizing an installation. Only use
the unpacked versions if they exist; otherwise fallback on the packed
versions.
Also filter the .woff files to make sure they are actually .woff files,
since they may be files like fonts.dir, fonts.scale, .uuid (created by
xorg-mkfontdir, xorg-mkfontscale, fontconfig).