Fix loading OS AppStream catalog data if it is available as YAML
Sometimes MIME detection produces a different mediatype application/yaml (which seems to be the default on more recent systems), which makes GNOME Software not load YAML AppStream data on Debian and Ubuntu. This change adds the mediatype to the recognized list, so GNOME Software works again.
Unless there is more to this issue than just this, this change fixes https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1058924 (I tested it, and with the patch applies, OS applications show up again).