This file is indexed.

/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.

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#!/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