mime type by file extension
A file extension is metadata, not the file itself: ".pdf" at the end of a name does not guarantee the bytes inside are actually a PDF, it is just a convention operating systems and servers use to guess the type before opening the file. That same convention is not unique either, which produces cases where one extension maps to more than one MIME type depending on which table gets consulted.
The extension search returns the first registered entry, application/font-woff2; font/woff2 is the correct form after RFC 8081, but it sits second in the table.
All three extensions point to the same registered type; none of them produces "image/jpg", because that type was never registered with IANA.
The table also registers application/xml for the same content; the extension search returns text/xml because it appears first, but both types decode the same XML document.
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.
Because web fonts were originally registered under the application/ top-level type, producing application/font-woff2, and only in 2017 did RFC 8081 create the dedicated font/ type, producing font/woff2. Older servers still declare the legacy form, so both stay in use and both get decoded by browsers.
No. The extension is chosen by whoever saves the file and can be changed freely without altering a single byte of the content; a .jpg file could contain anything. Servers accepting uploads should inspect the first bytes of the file (magic numbers) and never trust the name alone to decide how to process it.
No, both (and the rarer .jpe variant) resolve to the same registered type, image/jpeg. The three-versus-four-letter difference is just a historical leftover from the old FAT/DOS three-character extension limit, which never existed in the image format's own specification.
Search by file extension or full MIME type