Support embedded image in attachments

[thanhbinh01234]

A lot of email clients block remote images by default to avoid tracking. Being able to use images directly from attachments (via CID) would be really useful. But currently Plunk does not support it:

• Emails are sent only as multipart/mixed or multipart/alternative; inline images require content-type multipart/related.
• There’s no way to set a Content-ID on attachments.

It would be great to support multipart/related and allow attachments to have a Content-ID so they can be referenced in the HTML (e.g. cid:image1).

[5h0ov]

@driaug can I be assigned to solve this issue?