Why can't I import MXF files into iMovie on High Sierra, but I could on Sierra?

I recently updated my system on my MacBook Pro to High Sierra. However, I can no longer import MXF files from my Canon XF to iMovie 10.1.6. I'm running Sierra with imovie 10.1.6 on a Mac Mini in my classroom and I STILL can import MXF files into imovie. Anyone know why the update doesn't allow this?

I know I can just convert them using a program, but it's just a hassle for quick videos in Imovie. Is it possible to revert back to Sierra on my MacBook without screwing with any vital programs I'm currently running?