Thumbnail not updated

#3 work like a charm

The code change in the attached patch will trigger thumbnail logic every time media entity is saved. As that can be a slow process (some media types download thumbnails form 3rd party services) this isn't desired.

We should have some mechanism to detect or manually trigger thumbnail setting when needed.

This is a duplicate of #2786881: Recalculate thumbnail size during update

We don't know beforehand if a media type plugin provides local image or it has to download a remote image first. Should the media type plugin, in case of remote image, check the media first and avoid to download again a remote thumbnail? This is a behavior change but not quite BC break, only a performance penalty when media type plugin doesn't perform the check.

An regression is that there will be no way to update thumbnail for media entities with remote thumbnail. This is a hidden feature (not exposed in UI) and we can open a less important issue to fix that.

We worked around this by enabling the Thumbnail field for the form part of the field UI:

/**
 * Implements hook_entity_base_field_info_alter().
 */
function MYMODULE_entity_base_field_info_alter(&$fields, EntityTypeInterface $entity_type) {
  if ($entity_type->id() == 'media') {
    // Allow uploading or removing the thumbnail image.
    $fields['thumbnail']->setDisplayConfigurable('form', TRUE);
  }
}

Doing so allowed us to display the thumbnail within the media edit/create form, where the assigned image file could be removed, so it gets re-populated on save. This also allows to manually upload a thumbnail, if required.

Might be a solution for images: #2850169: Add proper image formatter for display current image

Marking this as duplicate of #2850169: Add proper image formatter for display current image and #2786881: Recalculate thumbnail size during update, please re-open if you believe this is not the case.