/lib/partman/valid_filesystems/05ext4 is in ubiquity 2.18.7.
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 21 22 23 | #!/bin/sh
dev=$1
id=$2
property=$3
[ -f /var/lib/partman/ext4 ] || exit 0
case $property in
formatable)
echo ext4
;;
existing)
[ -f $id/detected_filesystem ] || exit 0
fs=$(cat $id/detected_filesystem)
case "$fs" in
ext4)
echo ext4
;;
esac
;;
esac
|