chatty-file-item: Don't assume mime type is set

This e.g. isn't the case for matrix downloads as the mime type isn't yet known.

This fixes a crash when saving any attachment in a matrix chat.

Fixes 4e46e701 ("file-item: name vcard and vcal after contact and event name if saving")

I'm aware that @devrtz is working on making the whole download experience nicer but fixing the crash right away would be nice.