I wrote a patch and send it to bugzilla.
It works fine on my environment at least.
>> Sorry, but you can't. Currently the procedure that opens the thumbnail
>> can only specify the size of the original image. We could add the
>> possibility to optionally the specify number of layers. Currently it is
>> not possible though.
> I see.
> If I can write a patch, I will send it to bugzilla...
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