postinst is in leafpad 0.8.18.1-5.
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 | #!/bin/sh
set -e
if [ "$1" = configure ]; then
    # Register leafpad as a gnome-text-editor in the alternatives system.
    update-alternatives \
	--install /usr/bin/gnome-text-editor \
	    gnome-text-editor /usr/bin/leafpad 40 \
	--slave /usr/share/man/man1/gnome-text-editor.1.gz \
	    gnome-text-editor.1.gz /usr/share/man/man1/leafpad.1.gz
fi
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