/usr/share/mate/ui/drivemount-applet-menu.xml is in mate-applets-common 1.20.1-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | <menuitem name="Help" action="Help" />
<menuitem name="About" action="About" />
|