Some CCTV systems (like those from Amcrest or Swann ) use long numeric strings for saved clips.
If it won't open, use VLC Media Player —it plays almost any MP4 regardless of the name. 3753mp4
If this is a video you downloaded (like a movie or tutorial), you might need a guide on how to play , convert , or repair it. Some CCTV systems (like those from Amcrest or