This is Sushi, a quick previewer for Nautilus, the GNOME desktop file
manager.

Sushi is a DBus-activated service. For Sushi to popup, applications will
have to call the ShowFile method on the org.gnome.NautilusPreviewer 
interface. If you want to or debug Sushi without a calling application, 
you can use gdbus(1) to call the ShowFile method manually on the bus.