postinst is in sensible-utils 0.0.9.
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 | #! /bin/sh
set -e
case "$1" in
    (configure)
    if which update-mime >/dev/null;
    then
	    update-mime
    fi
    ;;
    (abort-upgrade|abort-remove|abort-deconfigure)
    ;;
    (*)
        echo "postinst called with unknown argument \`$1'" >&2
        exit 1
    ;;
esac
 |