multipart/form-data

An HTML form in separate parts, one per field. What a form posts when it uploads a file.

Extensions
None. It is not kept in a file of its own.
Header
Content-Type: multipart/form-data; boundary=7MA4YWxkTrZu0gW
Registered
Yes, in the IANA media type registry.
References
IANA · RFC 7578 · MDN

Common multipart types

Every media type, in one table