[FEATURE] Ability to write to Media's "thumbnail"

[rosiel]

Overview of feature request

It'd be really cool if we could use multifile media to write thumbnails - such as for PDFs and videos - that would take the place of the "default thumbnails" that show up in the Media list. It seems to be some kind of built in thing to Media.

We can't right now because it's not a real field, and doesn't have field_storage (so the processing we do before sending out the event fails).

Some modules seem to do this already - see

https://www.drupal.org/project/media_thumbnails_pdf
and its parent module https://www.drupal.org/project/media_thumbnails_pdf

What kind of user is the feature intended for?

Makes things really nice for end users, and for collection managers.

What inspired the request?

Trying to use PDF thumbnails in a site side-by-side with image media where we are making use of Drupal's media thumbnail.

What existing behavior do you want changed?

none?

Any brand new behavior do you want to add to Islandora?

Can write derivatives to the thumbnail not-a-field thing.

Any related open or closed issues to this feature request?

• kinda related to Drupal makes its own thumbnails, and it's confusing. documentation#1108
• definitely related: Thumbnails for collections, audio, pdf and video documentation#1111 - though this, afaik, means doing thumbnails in the traditional create-a-new-media sense.