postinst is in djview4 4.10.5-1.
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  | #! /bin/sh
set -e
case "$1" in
    configure)
	update-alternatives --quiet \
	    --install /usr/bin/djview djview /usr/bin/djview4 40 \
	    --slave \
	    /usr/share/man/man1/djview.1.gz djview.1.gz \
	    /usr/share/man/man1/djview4.1.gz
	;;
esac
 |