Agree with the missile comments and the plastic mount not being appropriate for the auto environment. However, that's actually an excellent location from the perspective that it will always be available even if you've got cargo or gear back there. Fast availability is key, and so is the extinquisher being where it always is no matter whether you have gear or whatnot aboard. In a panic, you'll go right to it. I've looked around a bit for a spot and there is simply nothing forward of the rear edge of the middle row that is not objectionable due to getting in the way of passenger usage, or directly striking a passenger in an accident.
If you like the location, perhaps a tether could be added that would hold back the missile effect. What comes to mine is a pair or three quality stainless hose clamps around the extinquisher with a cable run through them and thus clamped well. A steel loop is bolted onto the seatbelt bolt and the cable loops through a hole in the loop. One end of the cable has a climbing style rated locking carabiner that locks the cable into a loop. In this way, a severe crash would break the plastic mount, but the cable would be stout enough to control the extinquisher in a small swing radius. Unfortunately, part of the swing radius is going to blow the side cargo glass out, but if you're unconscious at the wheel of a wreck starting to smoke, having an exinguisher hanging out for all to see might be a good thing, eh? Obviously, the carabiner would then function as a quick release to get the extinquisher in service.
DougM