It's named "image/x-panasonic-raw2" only in shared-mime-info while programs that can handle the files specify "image/x-panasonic-rw2". This will prevent applications from showing up in the "open with" dialog in every file browser.
I tried to open a .rw2 file in dolphin with rawtherapee. I found out that also exiv2, ufraw, libopenraw (and others. Google will find them all) use x-panasonic-rw2.
It's been named that way since 51cbde9068b6528af8b01207c11172b72d78cc35, in November 2010, so I really don't see the point in changing it.
Why do those programs make up their own mime-types? Do they ship mime-type definitions instead of using shared-mime-info's?
Show me image/x-panasonic-rw2 being used before November 2010, and I'll make image/x-panasonic-rw2 the main mime-type. Otherwise, I'll just add image/x-panasonic-raw2 as an alias for the "correct" (read, oldest), mime-type.
exiv2 uses x-panasonic-rw2 in their wiki since Okt.1 2010:
Author: Bastien Nocera <firstname.lastname@example.org>
Date: Tue Mar 7 11:03:57 2017 +0100
Rename Panasonic RAW image mime-types to image/x-panasonic-rw*
As used in exiv2 since October 2010, a couple of weeks before the
mime-type was added to shared-mime-info:
This was likely a typo in the original definition.