[contextfree]

I’m not sure what was happening with his MD file extension, but I don’t think it could be what he thought it was. Windows 8 has only one unified set of file type associations – there’s no concept of desktop versus “Metro” there. Probably the app just didn’t register for that extension?

[freehunter]

That's a fair point. I know 7Zip has this same problem; no matter which computer I'm using, I have to manually tell Windows that 7Zip is the program to use to open a .7z file.

It's possible for a program to not register file extensions.