Here's the specification and here's the reference implementation C++ library lib3mf on Github. I'm about to file an intent-to-package (ITP) bug for lib3mf on Debian.3MF Import/Export
The 3D Manufacturing Format (3MF) as defined by the 3MF Consortium is one of the contenders to take over from STL. With support for extended features like colors, materials, properties, multiple volumes and the possibility to extend with both public and private features, it's much more versatile than the (for 3d printing) commonly used STL format. As additional reason for this format being a very attractive option is that its Specification and reference implementation are freely available and useable in Open Source projects.
This would be an interesting file format to add support for.