In MastoAPI media descriptions are updated via the
media update API not upon post creation or post update.
This functionality was originally added about 6 years ago in
ba93396649 which was part of
https://git.pleroma.social/pleroma/pleroma/-/merge_requests/626 and
https://git.pleroma.social/pleroma/pleroma-fe/-/merge_requests/450.
They introduced image descriptions to the front- and backend,
but predate adoption of Mastodon API.
For a while adding an `descriptions` array on post creation might have
continued to work as an undocumented Pleroma extension to Masto API, but
at latest when OpenAPI specs were added for those endpoints four years
ago in 7803a85d2c, these codepaths ceased
to be used. The API specs don’t list a `descriptions` parameter and
any unknown parameters are stripped out.
The attachments_from_ids function is only called from
ScheduledActivity and ActivityDraft.create with the latter
only being called by CommonAPI.{post,update} whihc in turn
are only called from ScheduledActivity again, MastoAPI controller
and without any attachment or description parameter WelcomeMessage.
Therefore no codepath can contain a descriptions parameter.
94e9c8f48a