Is it part of an or a specific digital art project?
: The identifier 13721 is synonymous with the Chrome Zero-day Vulnerability from 2019. In technical circles, an "mp4" suffix might be used to describe a proof-of-concept video showing the exploit in action, or a malicious file disguised as media to trigger such a bug.
did you find this file or name? (e.g., a specific forum, a dark web mystery, or a personal device) What are the contents of the video if you've seen it?