I have set up Hazel in a way so that movie files are moved to a different folder once the downloading has finished. But for some reason Hazel doesn't recognize .mkv files. It works fine with avi files for example.
How files are categorized is determined by the apps that handle them. mkv files aren't really native files so whether it is considered a movie depends on the app you have installed that handles those files. If you don't have such an app installed, then the system will have no idea what mkv files are. In that case, install such a program and it should work after that.
Not sure what you mean. I have VLC player installed and all mkv files opened immediately with that app. Hazel still doesn't recognize it. I tried kind = movie as well as extension = mkv and nothing happens