The difference between video/mp4 & video/x-mp4 & x-video/mp4

Did the usual google searches and can't seem to find any explanation of when to use what mime type. The best I can tell is it seems to designate streaming?! But why are there three 开发者_如何学Coptions? http://www.playstation.com/manual/psp/rss/en/spec.html (scroll down a bit)

Is it a matter of finding out what MIME type the file is? If so, how do I do that as, again, a google search doesn't give me any simple way of working that out!

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The correct MIME type is video/mp4 in all cases.

Some software such as that used by PSP blanket-allows x- prefixed versions of the MIME media type or subtype. There have been a number of subtypes that started out as x--prefixed unofficial versions before being blessed by IANA, so it does make some sort of sense, even though in this particular case neither video/x-mp4 nor an x-video type has ever existed. This is what the linked manual is documenting, but you wouldn't want to rely on it.