Image Alt Text: "After downloading a 2.5GB movie
Me: Presses play Movie unsupported file" A person is shown with eyes on her laptop punching the wall beside her, causing it to crack.
Mp4 and mkv are container formats. Your comment is somewhat equivalent to "where are you guys getting text files that can't be displayed, I've only ever seen zip files".
The codec in the container is what needs to be played, and can fail to render correctly.