Ensure it ends in .mp4 . If it doesn’t, try manually adding it to see if a media player can recognize the header.
Cloud storage services or private servers often rename files to unique IDs like 9vf9d4de10144 to prevent direct link guessing. Erox In 9vf9d4de10144 mp4
If the file won't play, it may be a "fragment" of a larger download that was never completed. Ensure it ends in