Add application-octet-stream, we need this one as last fallback.
Rename x-pdf to application-pdf as that is officially specified by IANA,
and also used by the fd.o shared mime db (and in consequence, by KDE/GNOME).

free desktop style icon naming in the pipeline, but I am going to wait till another version to add it
so I don't mess up existing sites running on the RC any more than necessary.