Are you trying to this file on your computer, or are you looking to replace it with a different video?
In the game's code, this MP4 file plays as a short, humorous video during a specific scene. Players often discuss this file because it can be in the game's local files to change the video that plays during that "feature". Managing MP4 Files
: Most operating systems can play MP4 files natively using Windows Media Player or QuickTime .
You can replace the mp4 file for the feature presentation : r/Undertale