postinst is in xpilot-ng-client-sdl 1:4.7.3-2ubuntu2.
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 | #!/bin/sh
set -e
if [ "$1" = configure ]; then
    # Add an alternative for xpilot and xpilot-ng
    # - we prefer xpilot-ng-sdl over xpilot-ng-x11 and original
    #   (non-NG) xpilot for 'xpilot' as it is a superset of
    #   original xpilot functionality
    update-alternatives --install /usr/games/xpilot xpilot \
        /usr/games/xpilot-ng-sdl 30
    # - we prefer the sdl NG version over the x11 NG version
    update-alternatives --install /usr/games/xpilot-ng xpilot-ng \
        /usr/games/xpilot-ng-sdl 20
fi
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