Well there you go!
[5.1.3. Mixed Subtype
The “mixed” subtype of “multipart” is intended for use when the body
parts are independent and need to be bundled in a particular order.
Any “multipart” subtypes that an implementation does not recognize
must be treated as being of subtype “mixed”.
5.1.4. Alternative Subtype
The “multipart/alternative” type is syntactically identical to
"multipart/mixed", but the semantics are different. In particular,
each of the body parts is an “alternative” version of the same
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