multipart/mixed

Parts of different types in one body, as an email with attachments.

Extensions
None. It is not kept in a file of its own.
Header
Content-Type: multipart/mixed; boundary=7MA4YWxkTrZu0gW
Registered
Yes, in the IANA media type registry.
References
IANA · RFC 2046 · RFC 2045

Common multipart types

Every media type, in one table