There is only one file type that uses the .MC6 extension:
Poser Material Collection File
File used by Poser Pro, a program used for modeling and animating characters; stores a collection of materials, which are the graphics that can be used to texturize a model's skeleton with a real-world appearance; may also use the .MCZ extension.
Material collections are used for distributing materials in a packaged format. Once imported, they can be viewed with the Library palette in the software.
NOTE: Poser was originally developed by Smith Micro, but was sold to Bondware in June 2019.
Updated: January 9, 2020