http content-type header mime
Content-Type is the header that decides how an HTTP client interprets the response body before the file is even opened. This page covers the structure RFC 6838 defines for that header value, how the vnd. and x. registration trees show up in practice, and why the file extension is never the type itself.
The vnd. prefix marks a format registered by a specific organisation, here Microsoft's OOXML extension for text documents.
This is the default Content-Type of any HTML <form> with no enctype attribute, even though the x. prefix has been formally discouraged since RFC 6838.
The same extension also maps to font/woff2, the correct type post-RFC 8081; both values coexist because both show up on real servers.
A MIME type (or media type) is a two-part identifier, type/subtype, that tells the receiver what format a piece of data is in. Originally created for email (Multipurpose Internet Mail Extensions, RFC 2045), it is now ubiquitous in HTTP, where the Content-Type header defines how the browser interprets content.
The standard tree (no prefix) is for general-purpose types registered by the IETF or by broad consensus, like text/html. The vnd. tree is for proprietary formats registered by a specific company or project, like application/vnd.ms-excel; any organisation can register its own vnd. subtype without needing a full IETF standard.
It is not required. RFC 8259 mandates UTF-8 encoding for JSON by default, which makes application/json; charset=utf-8 redundant in practice, though harmless. text/html and other textual types with no fixed encoding standard still benefit from an explicit charset to stop the browser from guessing wrong.
It stops the browser from ignoring the declared Content-Type and trying to guess the real type from the file content, a behaviour called MIME sniffing that older browsers used to tolerate misconfigured servers. Without nosniff, a malicious upload disguised as an image can be executed as a script if its content looks like HTML.
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