preinst is in libgtk-3-bin 3.4.1-0ubuntu1.
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 18 19 20 21 22 23 24 | #!/bin/sh -e
set -e
if [ "$1" = install ] || [ "$1" = upgrade ] ; then
    if dpkg-divert --listpackage /usr/sbin/update-icon-caches | grep -q libgtk3.0-bin; then
      dpkg-divert --remove --package libgtk3.0-bin --rename \
          --divert /usr/sbin/update-icon-caches.gtk2 \
          /usr/sbin/update-icon-caches
      dpkg-divert --remove --package libgtk3.0-bin --rename \
          --divert /usr/share/man/man8/update-icon-caches.gtk2.8.gz \
          /usr/share/man/man8/update-icon-caches.8.gz
    fi
    dpkg-divert --add --package libgtk-3-bin --rename \
        --divert /usr/sbin/update-icon-caches.gtk2 \
        /usr/sbin/update-icon-caches
    dpkg-divert --add --package libgtk-3-bin --rename \
        --divert /usr/share/man/man8/update-icon-caches.gtk2.8.gz \
        /usr/share/man/man8/update-icon-caches.8.gz
fi
exit 0
 |