wallpaper: Setting multiple backgrounds using wallpaper.xml not working

[phako]

When I set the background via the portal to be a wallpaper.xml file containing references to multiple images, the background will stay black.

Apparently gnome-shell fails to identify the default name "background" as an xml file and bails out with
Failed to load background 'file:///home/jens/.config/background': Unrecognized image file format.

The issue here seems to be the extension, if I move the file to background.xml and point gsettings to that, it will work fine. Not sure if this is actually an issue in gnome-shell though

[phako]

Ok, this is partially caused by me not writing the XML declaration in the file, which then makes the MIME type guessing fall back to ASCII. Fixing that on my side makes it work after restarting gnome-shell.

However, there is some special casing for files ending with .xml for backgrounds in gnome-shell which might cause some issues