.mix files

I have a client who has a number of image files saved as .mix files, photoshop does not like or anything else I can find either, and he needs the images.

As I understand it, the .mix format was a container that held an image and was opened by some ancient MS program. I'm rolling the dice here to see if anyone else has encountered and solved this issue. Though this is not exactly photoshop, it impacts me as a practitioner and my client so any help is apprecizted.

Thanks to those who know more than me,

Joe