One package can contain different tracks
Two resources using an MP4 container can carry different video codecs, audio codecs or profiles. The shared suffix does not require both to work in the same browser. Conversely, a codec can appear in more than one suitable container or delivery arrangement.
For an illustrative library, fileA and fileB both end in .mp4. FileA plays and fileB fails. That outcome does not establish that MP4 support disappeared between attempts. Record the internal track configurations and the actual response representation before assigning the failure to the suffix.
Do not rename the packaging into a repair
Changing an extension changes the name, not the encoded media or container structure. Remuxing and transcoding are also different operations: the former reorganises suitable encoded tracks, while the latter changes an encoded representation. Neither should be recommended blindly for protected or unauthorised material.
A supported playback route may already provide another representation better suited to the device. Use the service's documented choice where available. A viewer should not need to construct a private media conversion pipeline simply because a generic description says a familiar container is supported.
Keep HTTP and media metadata together
Content-Type describes the returned representation, while codec parameters can make a media type more specific. The actual bytes must still match the declaration. A login page at a media-looking address is a different problem from valid container data containing an unsupported audio track.
For a useful report, retain the declared type, known container, audio and video codec details, browser version and which component failed. Sound-only and picture-only outcomes can help narrow the track that needs investigation. Avoid publishing a universal compatibility table from extension names alone; it would omit the configuration that determines whether the actual media is usable.
Questions about this guide
Does .mp 4 name one fixed video codec?
No. MP4 can package different track configurations.
Does renaming a file transcode it?
No. A filename change does not alter encoded media.