Ensure the file extension is strictly .mp4 . If the file does not open, it may be corrupted or require a specific codec update. MP4 files explained: How to open and use - Adobe
For those interested in the technical differences between mobile-friendly formats, resources like Cloudflare’s video guide explain why MP4 is superior to formats like MOV for portable use.
Often cited as the best free MP4 player, VLC can handle almost any codec variation that might be used in a portable file.