postinst is in mythbuntu-live-autostart 0.69.
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 | #!/bin/sh
set -e
. /usr/share/debconf/confmodule
case "$1" in
        configure)
        for file in /usr/share/applications/mythtv.desktop \
                    /usr/share/applications/mythtv-setup.desktop \
                    /usr/share/images/xsplash/bg_2560x1600.jpg \
                    /usr/share/mythbuntu/Wallpaper-2560x2048.png;
        do
            if [ -f "${file}.diverted" ]; then
                dpkg-divert --package mythbuntu-live-autostart --remove \
                            --rename --divert ${file}.diverted ${file}
            fi
        done
        ;;
esac
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