Oleg Mitiukhin
Yes, musdb can only enter a placeholder there. The rendering of the 3D object is done entirely on the client side (say, the browser), and for generating the thumbnails we would need rendering capabilities on the server side. But rendering may take a long time and a lot of resources (say, chance it it would cause a timeout with larger files) and of course introduced a whole lot of code to maintain. In summary I don't think the benefits outweigh the costs.
But I just had an idea that may be useful. Maybe we could determine a fixed file name in the zip archive so that you could pregenerate the thumbnail. Say, if an image thumb.jpg existed in the root level of the zip, that could be used as the thumbnail and then automatically removed from the zip archive so as to save bandwidth for people viewing the object. Would that help?