The address does not define the body
A media client receives response metadata and bytes. If a request expected a playlist but receives an account login form, the problem is not repaired by renaming the response file. The representation itself is different from the required media description.
A 200 status says the server successfully handled that request under its chosen semantics. It does not independently certify that the response contains video, a valid manifest or the viewer's intended programme. Inspecting response type and a safe small sample of text can distinguish a login page from playlist syntax.
Type and coding name different layers
A content type such as video/mp4 describes a representation format. It does not fully declare which audio and video codec configurations appear inside the container. Content-Encoding, meanwhile, can describe an additional coding such as gzip applied to the representation for transfer.
For example, a gzip-coded text playlist remains a playlist after that coding is decoded. Gzip is not its video codec. A request inspector can show encoded transfer size and decoded resource size separately; comparing either number with an encoded video bitrate without checking the units creates misleading conclusions.
A practical three-part check
For an authorised support investigation, compare the expected resource, returned Content-Type and actual representation. A playlist should parse as the intended playlist format; a media segment needs appropriate media structure. Browser sniffing or one player's tolerance can hide a misconfigured header that another implementation rejects.
Publishers should serve accurate types for their actual resources. Viewers should keep the exact parser error and response status without posting private playlist contents. Successful parsing is still only one stage: nested requests, decoding and output remain necessary before the programme can appear.
Questions about this guide
Does .mp4 prove a response contains playable MP4?
No. Verify the returned representation and its actual media configuration.
Is Content-Encoding the video codec?
No. It describes an additional HTTP representation coding, such as gzip, rather than the programme’s video compression format.