Preview Plugin: Ability to manage files

**Description**
Currently the plugin only shows files that are already linked to a KG Resource.
Let's add the ability for a user to manage files:
- upload of new files and automatic linking to underlying resource
- removal of existing files.

This task includes the design of the UI update and validation prior to implementing.

TODO
- discuss whether images should be presented in this plugin considering that the Image Plugin already does it.

Start by locating the Preview Plugin entry point and its existing handling of files linked to a KG Resource. First resolve the UI design and image-plugin scope, then define validation for uploading and removing files; done means new files upload and link automatically, existing files can be removed, and the agreed UI behavior is covered.

Written by the indexing model from the issue text.