If cnxml is missing, raise a NotFound rather than server error

This gives much better information to the end user, compared to just
telling him that there's a server error
Post binary, watch out for unicode

Unicode filenames causes the request to break. Just use ascii, as the filename
doesn't really matter that much. Don't pass full path with upload (only
filename). do a binary upload rather than base64, it appears the JOD server
doesn't like base64 so much.