Modify child content type in hook_data

[michaeltalyansky]

I am trying to convert text-only emails to HTML. These emails are Content-Type: multipart/mixed, and text comes as Content-Type: text/plain in the child. I would like to be able to change to Content-Type: text/html, and modify body with add_body_filter.

As far as I can see, hook_data provides me with ability to see the content type in the child, but not change it. In the hook_data_post it is too late.

Describe the solution you'd like Would it be possible to provide in hook_data a callback with parameters like function (ct, enc, buff, headers) (where headers are the ones from the child, not from the email headers, which is transaction.body.header.get gives), so I can manipulate the headers I need and have them propagated to the child correctly?

Describe alternatives you've considered So far, I am at a loss about how to do this.

Additional context Email such as:

-> X-Mailer: swaks v20201014.0 jetmore.org/john/code/swaks/ -> MIME-Version: 1.0 -> Content-Type: multipart/mixed; boundary="----=_MIME_BOUNDARY_000_8111" -> -> ------=_MIME_BOUNDARY_000_8111 -> Content-Type: text/plain -> -> aaa -> -> ------=_MIME_BOUNDARY_000_8111-- ->