Hi, it uses webm, which I believe is the only video file type the MZ (editor) supports for things like selecting the file from the editor UI. I do have [CGMZ] File Types (https://casper-gaming.itch.io/cgmz-file-types) to allow you to use mp4 videos too, but it is a bit of a hacky workaround, imo it is easier to just use webm / convert to webm if possible.