This file is indexed.

postinst is in gnome-session 3.2.1-0ubuntu8.

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.

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#!/bin/sh

set -e

case "$1" in
  configure|upgrade)
    if [ -x /usr/lib/lightdm/lightdm-set-defaults ] ; then
        /usr/lib/lightdm/lightdm-set-defaults --keep-old --session=ubuntu || true
    fi
  ;;
esac