Bug description:

If a document has a field "electornicLocator" of any kind, this prevents the thumbnail to be loaded from the thumbnail service.

Expected behavior:

Only an "electronicLocator" of type "relatedResource" with value "coverImage" should replace the thumbnail service.

Steps to Reproduce:

1. Edit any document with a thumbnail
2. Add a field "electronicLocator"
3. See that the thumbnail is not loaded anymore