[CentOS] t-bird annoyance

Anyone got any clues? I'm running 6.3, t-bird 10.0.12, and KDE. I get
emails with .docx attachment, and t-bird has no idea what to do with them
(and doesn't even offer me a choice)."Change your preferences...

So I open the source of the email, and at the beginning of the doc I see
Content-Type:
application/vnd.openxmlformats-officedocument.wordprocessingml.document;

In system settings->advanced->file associations, I see the
applications/vnd... is set for libreoffice.

T-bird is still clueless.

      mark