multipart/mixed

The generic bundle: several independent bodies, each with its own media type, glued together by a boundary string. Email is its natural habitat — a text message plus attachments is multipart/mixed — and batch HTTP APIs borrowed it to pack several requests into one round trip. The parts are ordered but unrelated; when they are alternative renderings of the same content, multipart/alternative is the correct subtype instead. Parsers must take the boundary from the Content-Type parameter, never guess it from the body.

Defined by RFC 2046 · IANA registration. Registry facts from the IANA media-types registry via mime-db.