Old Video Formats for the Chop
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Full lists of the formats that will be supported and not supported are on an Apple Support page. If you have important elderly media files then now is the time to convert them to some more modern format; just make sure that you minimise any loss of quality by careful choice of the new format.
I'm personally sorry to see the demise of JPEG 2000, an improved version of JPEG that I don't think anyone ever used.