postinst is in lightdm-gtk-greeter 1.1.5-0ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | #!/bin/sh
set -e
if dpkg-maintscript-helper supports mv_conffile; then
  dpkg-maintscript-helper mv_conffile \
    /etc/lightdm/lightdm-gtk-greeter.conf \
    /etc/lightdm/lightdm-gtk-greeter-ubuntu.conf \
    1.0.0-0ubuntu1 -- "$@"
fi    
if [ "$1" = "configure" ] ; then
  update-alternatives --install /etc/lightdm/lightdm-gtk-greeter.conf \
    lightdm-gtk-greeter-config-derivative \
    /etc/lightdm/lightdm-gtk-greeter-ubuntu.conf 10
fi
case "$1" in
  configure|upgrade)
    /usr/lib/lightdm/lightdm-set-defaults --keep-old --greeter=lightdm-gtk-greeter || true
  ;;
esac
exit 0
 |