Media Library - Generate thumbnails on custom processors

Thumbnails are automatically generated on default processors since they are displayed on the media library index page, ie: they are required in order to make that page work correctly. But custom processors don't get to generate thumbnails automatically, they would need to call private APIs to generate one.

So we need to try generating thumbnails for all uploaded assets and maybe expose a public function so users can change the default behavior, for ex to customize thumbnails for videos and others types besides images.

No files or tests are named. Start by tracing how default processors generate thumbnails for assets shown in the media library, then compare the path taken by custom processors. Done means uploaded assets receive thumbnails without private API calls and the default behavior can be customized for videos and other non-image types.

Written by the indexing model from the issue text.