/usr/lib/mate-applets/ is in mate-dock-applet 0.70-1build1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/mate-applets/mate-dock-applet/
/usr/lib/mate-applets/mate-dock-applet/dock.py
/usr/lib/mate-applets/mate-dock-applet/dock_about.py
/usr/lib/mate-applets/mate-dock-applet/dock_applet.py
/usr/lib/mate-applets/mate-dock-applet/dock_color_changer.py
/usr/lib/mate-applets/mate-dock-applet/dock_custom_launcher.py
/usr/lib/mate-applets/mate-dock-applet/dock_prefs.py
/usr/lib/mate-applets/mate-dock-applet/dock_win_list.py
/usr/lib/mate-applets/mate-dock-applet/dock_xml.py
/usr/lib/mate-applets/mate-dock-applet/docked_app.py
/usr/lib/mate-applets/mate-dock-applet/dom_color.py
/usr/lib/mate-applets/mate-dock-applet/log_it.py