This is a small set of custom thumbnailer scripts (mostly for 3D printing formats) that I crafted based on various samples floating about on the Internet, and which work in GNOME 42+.

In short, these will render pretty little file previews in your file manager (not just Nautilus, but that is what I tested with):

• .3mf (Cura, PrusaSlicer, SuperSlicer, etc.)
• .gcode with embedded thumbnails
• .scad (OpenSCAD, will blindly render the default object)
• .stl (Actually uses OpenSCAD to render as well)
• .blend (uses the built-in blender-thumbnailer that comes with Blender)

• put .thumbnailer files in /usr/share/thumbnailers
• put the other files in /usr/local/bin, marked as executable
• remove all the cached thumbnails in ~/.cache/thumbnails
• restart Nautilus with nautilus -q

The Makefile does that for you.

• 3mf.thumbnailer and gcode.thumbnailer require nothing but Python 3 installed, because they cheat and use the thumbnail the slicer inserts into the file.
• stlscad.thumbnailer requires Xvfb, openscad and ImageMagick installed to render and convert files.
• blender.thumbnailer requires Blender to be installed.

But isn't openscad prevented from accessing X because of bwrap thumbnail generation sandboxing in modern GNOME?

Yes. But it works perfectly with xvfb-run and software rendering, so I'd rather take the performance hit rather than spend hours trying to persuade bwrap to do weird stuff like --bind /tmp/.X11-unix/X0 /tmp/.X11-unix/X0 --setenv DISPLAY :0.

Sometimes brute force is the simplest, more maintainable (and more secure) approach.

I have a .3mf or .gcode file without thumbnail, is the thumbnailer broken?

No. It's quite likely that the slicer you used to create it did not include a thumbnail, or that it is in a weird format. There are limits to these things.

My OpenSCAD thumbnails are blank!

This usually happens when it's either a library file (that would render nothing on its own) or a customizer file that is missing some default (and hence renders nothing as well).

My Blender thumbnails are blank!

That usually happens when you either don't have a camera defined or it's pointing towards literally nothing.