Support embedded images

[leebenson]

Goals

I realise this is potentially difficult to render an could be seen as more of a sender concern rather than at the HTML/plain text level, but have you given any thought to allowing for embedded images that are referenced by CID?

This would help a particular use-case where I'd like for messages to be self contained rather than requiring external hosting.

Background

Image embedding is somewhat contentious. It increases the size of the email message and doesn't explicitly solve for image blocking, but for certain use-cases (particularly for dynamic images or where emails may be rendered differently depending on props), it can make for compact and self-contained messages that don't require external hosting.

Proposal

I think this would have to be tackled in several ways: potentially an <EmbedImg> component and render support that could load the URL/local file, create a content boundary in the resulting HTML, and swap it out for a CID ref.

There are similar requests such as #870, but I don't think a Data URI is the way to go due to the lack of client support.