/usr/share/nemo/actions/set-as-background.nemo_action is in nemo-data 2.2.4-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 | [Nemo Action]
Name=Set as Wallpaper...
Comment=Set the selected image as your desktop wallpaper
Exec=gsettings set org.cinnamon.desktop.background picture-uri "%U"
Selection=s
Mimetypes=image/*;
Conditions=dbus org.Cinnamon;
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