MeshMixer

AutoDesk MeshMixer uses a native binary format with the extension .MIX. Meshmixer is free software released in 2009.

File Information
MIX files seem to have XML starting at offset 0x4 with the tags:. Then file is a binary mesh format which seems to be related to "GSurf" & "VFMesh".

Newer MIX files may not be backwards compatible with older versions.

Software & Sample Files

 * Autodesk Meshmixer API → https://github.com/meshmixer/mm-api/blob/master/python/examples/sphere.mix
 * Instructables Sample → https://content.instructables.com/ORIG/FMQ/6ES2/HPXFS5LS/FMQ6ES2HPXFS5LS.mix