Separate packaging from compression
A container can hold a video track, one or more audio tracks, subtitles and other information. The video and audio can use different codecs. Supporting the container does not automatically mean supporting every possible track combination inside it.
This explains a common symptom: a player can open an item and play its audio while failing to display the video. The container was readable, but one part of the media path may still be unsupported or failing.
Do not use renaming as conversion
Changing a filename extension does not re-encode the tracks or restructure the container. A device may select a different handler, but the underlying data remains the same. Treat a renamed file that happens to open as a compatibility observation, not a general solution.
For streaming, the player may also need to understand a manifest or protocol before reaching the media. A playlist loading successfully therefore answers only part of the compatibility question.
Collect the details that matter
Where the player exposes media information, record container, video codec, audio codec, resolution and frame rate. Include profile or level information when the support tool shows it. Avoid guessing those values from a marketing label.
Check the exact device and app documentation. A manufacturer’s statement about hardware capability does not prove that every installed app uses it, and a browser’s support can depend on its platform.
Questions about this guide
Does MP4 support mean every MP4 will play?
No. The audio/video codecs and their configurations still need support in the actual player and platform.
Will changing an extension convert the video?
No. Conversion changes the media data or packaging; renaming alone does neither.