This file is indexed.

/usr/lib/os-probes/mounted/10freedos is in os-prober 1.76~deb9u1.

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

. /usr/share/os-prober/common.sh

partition="$1"
mpoint="$2"
type="$3"

# Weed out stuff that doesn't apply to us
case "$type" in
	vfat) debug "$1 is a FAT32 partition" ;;
	msdos) debug "$1 is a FAT16 partition" ;;
	fat) debug "$1 is a FAT partition (mounted by GRUB)" ;;
	*) debug "$1 is not a FAT partition: exiting"; exit 1 ;;
esac

if item_in_dir -q kernel.sys "$2" && item_in_dir -q command.com "$2"; then
	label="$(count_next_label FreeDOS)"
	result "$1:FreeDOS:$label:chain"
	exit 0
else
	exit 1
fi