Importing ATEM mp4 files into Avid Media Composer

I'm attempting to import or link files created by an ATEM mini ISO into Avid Media Composer 8.6.5 on an iMac. I get a dialogue box that says the file is not supported for import. It won't allow me to link either; I don't even get a dialogue box when it doesn't link. I think this is the first time that I've ever been unable to import an mp4.

I have no problems importing the .png and .wav files created by the ATEM.

Anyone else encounter this? Is there some kind of new, improved need-to-buy-a-new-computer mp4 nowadays? Or I am I being forced into using Resolve?

Barry