/usr/bin/2ff is in farbfeld 3-5.
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 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 | #!/bin/sh
# arguments
if [ "$#" -ne 0 ]; then
	echo "usage: $0" >&2
	exit 1
fi
# write input into temporary file
TMP=$(mktemp)
trap 'rm "$TMP"' EXIT
cat > "$TMP"
# determine the mime-type
if [ "$(dd if="$TMP" bs=1 count=8 2>/dev/null | tr -d '\0')" = "farbfeld" ]; then
	cat "$TMP"
else
	MIME=$(file -ib "$TMP" | cut -d ";" -f 1)
	case "$MIME" in
	image/png)
		png2ff < "$TMP"
		;;
	image/jpeg)
		jpg2ff < "$TMP"
		;;
	*)
		if [ ! -x /usr/bin/convert ] ; then
			printf "%s: cant convert from %s -- missing \`convert' program" "$0" "$MIME" >&2
			printf "Run \`apt install imagemagick' to install it" >&2
			exit 1
		fi
		convert "$TMP" png:- | png2ff
		;;
	esac
fi
# errors
if [ $? -ne 0 ]; then
	exit 1
else
	exit 0
fi
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