Other multipart subtypes are expected in the future. MIME implementations must in general treat unrecognized subtypes of multipart as being equivalent to "multipart/mixed".
The formal grammar for content-type header fields for multipart data is given by:
multipart-type := "multipart" "/" multipart-subtype
";" "boundary" "=" boundary
multipart-subtype := "mixed" / "parallel" / "digest"
/ "alternative" / extension-token