While there isn't a specific known "internet mystery" tied to the exact filename , the double extension usually signals a few specific technical or security situations. 1. Security Risk: Hidden File Extensions
If you are looking into this because you need to play or edit the file: 6010.mkv.mp4
If your computer is set to "Hide extensions for known file types," you might only see 6010.mkv , leading you to believe it is a standard video file. While there isn't a specific known "internet mystery"