/usr/share/gnome-main-menu/GNOME_MainMenu_ContextMenu.xml is in mate-gnome-main-menu-applet 1.8.0-5build1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | <menuitem name="MainMenuOpen" action="MainMenuOpen" />
<menuitem name="MainMenuAbout" action="MainMenuAbout" />
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