: Your computer might be set to "Hide extensions for known file types." If the original file was already an MP4, it would look like 9126354 . Adding .mp4 manually results in the double extension.
: Use the VideoWriter class. Ensure you specify the correct FourCC codec (like avc1 for H.264) to ensure compatibility. 9126354.mp4.mp4
When you see a file named 9126354.mp4.mp4 , it typically means: : Your computer might be set to "Hide