fix: use xxx as unknown extension (.123 has been registered)

the .123 extension has been registered:
    https://www.iana.org/assignments/media-types/application/vnd.lotus-1-2-3
6 jobs for !23 with topic/default/set_cubes_cw_max_version in 2 minutes and 30 seconds (queued for 8 seconds)
latest detached
Status Name Job ID Coverage
  Lint
passed black #530617
lint black

00:01:01

passed check-manifest #530618
lint check-manifest

00:00:16

passed flake8 #530616
lint flake8

00:00:23

passed safety #530620
lint safety

00:02:20

passed yamllint #530619
lint yamllint

00:00:21

 
  Tests
passed py3 #530621
tests py3

00:01:00