Thanks for the response. Yeah, I know it isn't really media. But it can be handled that way.
I had come up with both your solutions, great minds think alike. I used the Bulk renamer to fix the extensions, and then I use MC to catalog the images I have for each stl. It works, I can still make lists of favorite files and see them, I just have to "Locate External" to find the actual printable files. It works.
Thanks again for your reply.