No attachment Content-ID

[tholo]

Attachment objects does not generate, or easily allow one to specify a Content-ID: content-id header for the attachment part(s), which makes it so one cannot refer to them from an HTML body (think small images or thumbnails in HTML format email bodies, using <img src="cid:content-id" /> tags).

[tseaver]

Relevant specs:

• RFC 2387: The MIME Multipart/Related Content-type, http://tools.ietf.org/html/rfc2387
• RFC 2393: Content-ID and Message-ID Universal Resource Locators, https://tools.ietf.org/html/rfc2392

[tseaver]

I'd be glad to review a PR which added a 'content_id' parameter to Attachment.__init__, and then propagated to the header appropriately if passed. I think generating one randomly would be useless, as the message text would have no way to incorporate it.