This file is indexed.

/etc/network/if-down.d/upstart is in ifupdown 0.7.47.2ubuntu4.

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 -e

if [ -e /lib/lsb/init-functions ]; then
    . /lib/lsb/init-functions
fi

if ! init_is_upstart; then
    exit 0
fi

# Let's ignore meta entries (ifdown -a)
if [ "$ADDRFAM" = "meta" ]; then
    exit 0
fi

initctl emit -n net-device-down \
	"IFACE=$IFACE" \
	"LOGICAL=$LOGICAL" \
	"ADDRFAM=$ADDRFAM" \
	"METHOD=$METHOD"