Ipx-550.mkv.mp4

If you need to change the actual format (not just the name) for better compatibility, use a tool like Handbrake :

Right-click the file and remove the unnecessary .mkv part so it simply ends in .mp4 . IPX-550.mkv.mp4

Download VLC Media Player or MPV . These players ignore extension errors and read the "magic bytes" inside the file to determine how to play it correctly. If you need to change the actual format