/lib/partman/mount.d/70jfs is in ubiquity 2.10.16.
This file is owned by root:root, with mode 0o755.
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  | #!/bin/sh
set -- $1
fs=$1
mp=$2
type=$3
options=$4
dump=$5
pass=$6
case $type in
    jfs)
	mount -t jfs ${options:+-o "$options"} $fs /target$mp || exit 1
	echo "umount /target$mp"
	exit 0
	;;
esac
exit 1
 |