Extract Thumbnail can't allow only one rgb channel

[moonyuet]

          I mean we could change the logic in extract thumbnail so it doesn't look only for rgba channels but if there is at least one, use it and copy it to rgb. We'll need to solve somehow how to deal with data that are not image (like point positions).

Anyway, I think this is work for another PR that would touch how thumbnail are generated in that case.

Originally posted by @antirotor in #6127 (comment)

[cuID:OP-7987]