Click Start Encode to create a clean, single-extension version.

If .mp4 doesn't work, try renaming it to end only in .mkv . 4. Conversion Guide

This often happens if someone manually renamed the file or if a conversion process failed to replace the old extension. Usually, the last extension ( .mp4 ) is what your operating system will try to use. 2. How to Play the File If your media player shows an error, try these steps:

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.