The orphaned Windows format
3MF was Microsoft's modern 3D-printing format — and Windows happily created such files in Paint 3D and opened them in 3D Viewer. Then Paint 3D was discontinued, and 3D Viewer was pulled from the Store. The files remain; the built-in ways to open them are gone.
This converter unpacks the 3MF (it is a ZIP with XML inside), reads every object in the build — including placement transforms, so multi-part files come out arranged exactly as designed — and writes a clean binary STL for your slicer or an OBJ for your editor. It also reports the triangle count and the model's real dimensions in millimetres, so you know whether it fits your printer bed before you slice.
3D models are often client work or unreleased designs — the kind of files that should not tour the internet. Here the conversion happens on your machine, full stop.