I've been looking at email Actions (which apparently are in a process of being standardized), and considering implementing these for an application of mine.
However, the entire documentation seems to be missing the mime-type to define for the message part that includes this json-ld metadata. For example, gpg signatures are marked as
Content-Type: application/pgp-signature; name="signature.asc"
What Content-Type does this part (eg: this content) need to be included as?