This is Nautilus 42.1.1-2 and papirus-icon-theme 22020508-1 - this install is a few days old.
My .stl files are getting the grey question mark unknown icon
Looking through Papirus’ git,
/usr/share/icons/Papirus-Dark/16x16/mimetypes/model-stl.svg exists
I also find a command to run;
gio info -a standard::icon
…
attributes:
standard::icon: unknown, model-stl, model-x-generic, unknown-symbolic, model-stl-symbolic, model-x-generic-symbolic
So why the unknown assumption? I’ve only got OpenSCAD & Cura on this machine at the moment, if that makes any difference.