gnome-session currently installs files/defaults.list-r6. It should be ported to mimeapps.list.
It already is installed as gnome-mimeapps.list, it's just historical naming which we can interpret as a gnome default for the *.list extension file that is for that :) But I guess if that's some legacy format that makes bugs filed, we can update the name of the file in git when it gets updated next or something. Definitely doesn't sound like a blocker for 926410 - GNOME was probably first to ship a distro provided mimeapps.list in Gentoo - maybe a decade or so ago ;)
Wait, on checking it over again, I'm confused. Is this actually about the folder we are installing this file to, not any format thing inside it?