application/x-www-form-urlencoded

What a plain HTML form submits: key=value pairs joined with ampersands, spaces turned into plus signs, everything else percent-encoded. It is the default body type for form posts and the shape of every query string. The encoding is the trap — it predates and slightly disagrees with ordinary URL percent-encoding (that plus-for-space rule), so decoding with the wrong function silently corrupts data. It carries flat strings only; files and binary payloads belong in multipart/form-data instead.

Defined by WHATWG URL Standard · IANA registration. Registry facts from the IANA media-types registry via mime-db.