I would like to add a menu item to the context menu that appears when you right-click a file in GNOME Files (nautilus).
This is unrelated to gnome-shell, so nautilus-list would be a better place to ask. However:
In older versions, you'd put a script in ~/.gnome2/nautilus-scripts/ (or something to that effect).
The location was changed to ~/.local/share/nautilus/scripts a couple of years ago, other than that I don't think there have been any changes to the scripting functionality (it's certainly still there) ...