Context

Providing thumbnails in search results was implemented in T250144 and T250207 .
At the moment there's no integration test that explicitly verifies correctness of representation of Thumbnails objects into JSON FileRepresentation.

the tests run on core checks result without extensions that provide details, so it thumbnails always null there.
PageImage extension implements this feature but doesn't contain integration tests

Expected Result

• Integration tests that verify proper serialization should be implemented.
• The way and place should be discussed.