multipart/related

The multipart for compound documents whose parts only make sense together: a root part plus the resources it references, with a required type parameter naming the root's media type and an optional start parameter locating it. RFC 2387 (1998) defined it, and its two great careers followed — HTML email that embeds its images as cid: parts instead of remote fetches, and the .mhtml single-file web archives browsers save. SOAP with attachments rode it too, back when that was a sentence people said. The eternal parser gotcha applies to the whole multipart family: the boundary lives in the Content-Type header, so a body separated from its headers is unreadable.

Defined by RFC 2387 · IANA registration. Registry facts from the IANA media-types registry via mime-db.