: Typically encoded with H.264/H.265 and AAC.

: By locating the game's local files and identifying the specific .mp4 associated with this "feature," users can swap it with their own video file (renamed to match the original) to have custom video play during that segment of the game. General MP4 Information

In the context of Undertale , particularly on PC versions, the game uses MP4 files for certain scripted sequences.