Now that you bring that up, I also had issues when loading MP4s after recently upgrading to HS. it is a known issue since MacOS 10.13.
It should of been fixed though.
What MacOS and MetaZ version are you using? I am using Version 1.0.beta-34 and it works fine. I will try the latest release and see if it works.
Click on the "MetaZ" on the menu bar and click "About MetaZ" to see the version.
[doublepost=1545755964][/doublepost]I just tried the latest beta-45 and it works fine.
It is just asking what type of media it is, like Movie, TV show, Home Movie.
In older versions, this information used to be set to a default, like movie for example, and if the video you were tagging was a TV show, then you had to manually change the media type to TV Show.
I actually like that it asked what type of media it is.