there is a little update on the subject, that may be useful to you and/or the developers.
In the message above I expressed a WISH, but now I actually think it's a bug, rather than something not implemented.
Why? Because if I have a bunch of BMPs in a folder, I can actually see their thumbnails. But for any other type of picture I only see a "static" icon (even if they have a thumbnail in the extended attributes).