postinst is in ubuntu-wallpapers 18.04.1-0ubuntu1.
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 | #! /bin/sh
set -e
case "$1" in
    configure)
        # Workaround LP: #746028
        touch /usr/share/backgrounds/warty-final-ubuntu.png
    ;;
    abort-upgrade|abort-deconfigure|abort-remove)
    ;;
    *)
        echo "$0 called with unknown argument \`$1'" 1>&2
        exit 1
    ;;
esac
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