In some internet subcultures, specifically named MP4 strings occasionally go viral as "screamers" (jump scares) or "shock videos." If you don't recognize the sender, it is best to avoid opening it. How to identify the content
If you have the file and want to know what it is without playing it:
A very small file (under 1MB) is likely a short meme or a GIF-style loop. 5125212941360038568.mp4
This specific numeric format is how Discord renames files uploaded to its servers. If you found this in your computer's cache or a shared folder, it likely originated from a Discord chat.
You can upload the file to an online metadata tool to see the "Media Created" date or the original software used to encode it, which can give a clue to its origin. In some internet subcultures, specifically named MP4 strings
When videos are downloaded via third-party bots (like "SaveVideo" bots), the original title is often replaced by a unique ID like this to prevent filename conflicts. Safety and Content Warning
Files with long numeric strings are frequently used to share memes, but they are also a common format for malware or phishing links. If you found this in your computer's cache
Because this is a generic system-generated filename, there is no single "official" video or review attached to it. However, Why this filename exists