I have lots of midi files created in Cubase VST on an old Macbook (long dead) more than ten years ago. I recently installed Cubase AI and tried to load a couple of these files without success. I tried appending .mid to the file names, but it made no difference. Are these files (lots of songs with carefully prepared scores) write-offs?
Are you sure, these are MIDI files? I would say, these are old Cubase project files (*.arr or *.all). Check this thread, please.
But, as an AI owner, you cannot use Cubase SX with your license. You should use Cubase SE instead of SX.