There is only one file type that uses the .MC5 extension:
Poser 5 Material File
Contains material or texture information used by Poser's rendering engine; can be applied to 3D models and offers a greater scale of effects than Poser MAT pose (.PZ2) files; only accessible from the Library within the Material Room.
Poser was originally developed by Smith Micro, but was sold to Bondware in June 2019.
Updated: June 14, 2020