/usr/share/lightdm/sessions/unity8-mir.desktop is in unity8-desktop-session-mir 1.0.12+15.10.20150609-0ubuntu1.
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 | [Desktop Entry]
Name=Unity8-Mir
Exec=lightdm-unity8-session
Type=Application
X-LightDM-Session-Type=mir
|