/lib/udev/rules.d/85-mdadm.rules is in mdadm 3.2.3-2ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | # This file causes block devices with Linux RAID (mdadm) signatures to
# automatically cause mdadm to be run.
# See udev(8) for syntax
SUBSYSTEM=="block", ACTION=="add|change", ENV{ID_FS_TYPE}=="linux_raid*", \
RUN+="/sbin/mdadm --incremental $env{DEVNAME}"
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