Distinguish the address from the response
A URL can end in .mp4, another extension or no extension at all. The server separately sends response metadata such as Content-Type. Mozilla’s browser documentation explains that browsers use the MIME type to determine processing. This means a successful file download and successful in-page playback are related but different observations. Keep the actual response type when investigating why a resource opens differently in two contexts.
Read the type at the right level
A type such as video/mp4 describes a media container category; it does not fully identify every codec and profile inside. Additional codec information may be needed for compatibility decisions. A correctly labelled response can still contain media the application cannot decode. Conversely, a familiar filename does not repair a response that is being delivered or interpreted under an unsuitable type. Inspect both rather than treating either as complete evidence.
Capture a small authorised diagnostic record
For a web playback problem, the application publisher can inspect the request status, response type and player error. A viewer does not need to publish a complete private URL to communicate those observations. If the response is an HTML login page rather than the expected media, report that distinction. Renaming the saved page as a video will not turn the returned document into playable audiovisual data.
Avoid local fixes that hide the actual issue
A file association on your computer controls which local application opens a file; it does not change the original HTTP response. Keep those settings separate from the server’s delivery behaviour. Report whether a resource downloads, opens in another player or fails only inside the browser. Each is a useful clue, but none alone proves that credentials, codecs or all server headers are correct.
Questions, answered
Does .mp4 at the end of a URL guarantee MP4 playback?
No. The response metadata, actual contents, codec combination and player support still matter. A filename is only one label in that chain.