Set Content-Id header on attachments through send message API

[memcorrupt]

Good to see that postalserver seems to have resumed development. 🙌

Currently, there is no support for setting the Content-Id header on attachments sent through /api/v1/send/message. I'm not quite well versed enough with the project & programming in Ruby to pull request it at this time, but from my brief look into the source code, I believe it can be implemented pretty easily.

1. Add cid parameter and handling to attachments in send_api_controller.rb, so that it gets passed to OutgoingMessagePrototype.new
2. Set content_id key within mail.attachments[name].
   • https://github.com/postalserver/postal/blob/main/app/models/outgoing_message_prototype.rb#L177
   • https://github.com/mikel/mail/blob/master/lib/mail/attachments_list.rb#L84

If there is any documentation that could ease the setup of a development environment for Postal, I could attempt to make the changes and PR them upstream.

[olegbliaher]

That would be a nice feature.