: It may simply be a standard MP4 video that has been poorly named (e.g., missing the dot before "mp4"). If you encountered this as a file on your device, it is likely a standard H.264 or H.265 video file that requires a media player like VLC to open.
: In many cases, files named with unusual extensions or suffixes like "video-18.mp4" or "video-18mp4" are found on unreliable streaming sites or in spam messages [1, 2]. These are often used to trick users into downloading malware masked as a video file [2]. video-18mp4
Where did you encounter this file name, and are you trying to or identify its source ? : It may simply be a standard MP4