STAC collection thumbnail images can be defined but not edited

[j08lue]

When I add a thumbnail image to a STAC collection, it gets added fine, but when opening that collection, the field is not filled and the property cannot be edited.

Steps to reproduce:

1. Open a collection without a thumbnail in STAC Manager
2. Add a thumbnail via the dedicated form fields
3. Save
4. Open the collection and confirm that the thumbnail shows up correctly
5. Verify on STAC API that the collection-level asset has been added correctly
   
6. Edit the collection
7. Note that the Thumbnail form fields are not filled and the added collection-level asset is not listed
   
8. Expected would be: The configured thumbnail shows up either in the collection-level assets or the dedicated form fields.