Header Mime Types are one of the oldest ways for the web to communicate what type of media a file is (I would assume file extensions are the oldest way). Mime types are strings of meta-data sent when HTTP is communicating what the content type of a file being transmitted is. But, with this meta-data categorization there has been differences between browsers and servers regarding mime type strings and their association. This type of overlaying, now-a-days obviously duplicating, information is pretty standard for the older web ways of doing things, especially if they are still used in various forms in todays web world.
A list of all mime-media types are available on the IANA website.
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