Set renderable preview if one wasn't set and is uploaded

[Floppy]

Discussed in https://github.com/manyfold3d/manyfold/discussions/3933

^{Originally posted by RJGhugo April 6, 2025} There is no "preview" when uploading first the FreeCAD file and second the STL file.

When uploading first the STL file, I am getting a "preview".

Is there a option to take alwas the "first" STL to generate the "preview"?

We could automatically set a renderable file to be the preview if one is added when there isn't a renderable preview already set. And, we could automatically do the same if an image is uploaded; if the preview isn't already an image, set the freshly uploaded one to preview.