You're on linux? The odds of you getting a virus on linux are not 0 but very slim, since the userbase is very small.
Plus, viruses prey on people's ignorance. The usual "movie file viruses" are .exe files and can only be run on windows. Most people don't enable the option to show file extensions on windows, so a filed named "movie.mkv.exe" would show up as "movie.mkv" instead.
IMO, the odds of you accidentally running a virus by playing a .mkv file on linux are as high as the odds of you winning the lottery 3 times in a row.