/usr/share/xfce4/panel-plugins/topmenu.desktop is in xfce4-topmenu-plugin 0.3-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | [Xfce Panel]
Type=X-XFCE-PanelPlugin
Encoding=UTF-8
Name=TopMenu
Comment=Shows the topmenu menu bar
Icon=topmenu-applet
X-XFCE-Module=topmenu
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