postinst is in nano-tiny 2.2.6-1ubuntu1.
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 | #!/bin/sh
set -e
if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ]; then
    update-alternatives --install /usr/bin/editor editor /bin/nano-tiny 0 \
      --slave /usr/share/man/man1/editor.1.gz editor.1.gz \
      /usr/share/man/man1/nano-tiny.1.gz
fi
 |