postinst is in libxml2 2.9.1+dfsg1-3ubuntu4.
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 # Automatically added by dh_makeshlibs if [ "$1" = "configure" ]; then ldconfig fi # End automatically added section