/usr/share/dbus-1/services/org.freedesktop.FileManager1.service is in nautilus 1:3.18.4.is.3.14.3-0ubuntu4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | [D-BUS Service]
Name=org.freedesktop.FileManager1
Exec=/usr/bin/nautilus --gapplication-service
|