Quicktime won't open HEVC video file on iMAC with High Sierra

I have a 5k iMac running High Sierra. It will not open HEVC video files, even though VLC will do it. Everwhere I read on-line it says a Mac with High Sierra should play these files with Quicktime. My Quicktime is Version 10.4 (928.13).

Is there a way for me to fix this so that Quicktime can handle these files. I have work arounds, but I shouldn't need to use VLC, or take the file to another Mac, etc..

I really don't want to upgrade the operating system just to do this - it'll set off a chain reaction of bigger issues, I'm afraid.