MIME type to satisfy HTML, email, images and plain text?

The answer to Mail multipart/alternative vs multipart/mixed suggests that attachments should be peers of the multipart/alternative message, like:

multipart/mixed
- multipart/alternative
  - text/plain
  - text/html
- some/thing (disposition: attachment)
- some/thing (disposition: attachment)
- ...

I'd like to send email with an html part with some inline images and a plain text alternative. What is the preferred MIME layout for the various parts? A couple of options appear in example code and in other questions, but which have worked best in practice? My inclination is this:

multipart/alternative
- text/plain
- multipart/related
  - text/html (referencing the images by cid)
  - image/gif
  - image/gif
  - ...

That way, the images are clearly for the purpose of rendering the html part. A full example of this would be:

From: Rich Example <rich-example@example.org>
To: A Recipient <recipient@example.org>
Subject: An example of email with images and a plain alternative
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="outer-boundary"

This is a MIME-encoded message. If you are seeing this, your mail
reader is old.
--outer-boundary
Content-Type: text/plain; charset=us-ascii

This message might make you :) or it might make you :(

--outer-boundary
MIME-Version: 1.0
Content-Type: multipart/related;
  type="text/html"; start="<body@here>"; boundary="inner-boundary"

--inner-boundary
Content-Type: text/html; charset=us-ascii
Content-Disposition: inline
Content-ID: <body@here>

<html>
 <body>
  This message might make you
  <img src="cid:smile@here" alt="smile">
  or it might make you
  <img src="cid:frown@here" alt="frown">
 </body>
</html>

--inner-boundary
Content-Type: image/gif
Content-Disposition: inline
Content-Transfer-Encoding: base64
Content-ID: <smile@here>

R0lGODlhEAAQAKEBAAAAAP//AP//AP//ACH5BAEKAAIALAAAAAAQABAAAAIzlA2px6IBw2
IpWglOvTahDgGdI0ZlGW5meKlci6JrasrqkypxJr8S0oNpgqkGLtcY6hoFADs=

--inner-boundary
Content-Type: image/gif
Content-Disposition: inline
Content-Transfer-Encoding: base64
Content-ID: <frown@here>

R0lGODlhEAAQAKEBAAAAAAD//wD//wD//yH5BAEKAAIALAAAAAAQABAAAAIzlA2px6IBw2
IpWglOvTahDgGdI0ZlGW5meKlci75drDzm5uLZyZ1I3Mv8ZB5Krtgg1RoFADs=

--inner-boundary--

--outer-boundary--

You are right. Inline images should be stored in a multipart/related mime-entity (RFC 2387) and offering multiple content-type options can be done with multipart/alternative (RFC 2046).
For adding attachments you may put the whole structure into a multipart/mixed and add the attachments.

multipart/mixed
- multipart/alternative
  - text/plain
  - multipart/related
    - text/html
    - image/gif
    - image/gif
- some/thing (disposition: attachment)
- some/thing (disposition: attachment)

You can also use inline image in text/plain messages, but not all MUA support this. (Use none or disposition: inline)

multipart/mixed
- text/plain (text above image)
- image/gif
- text/plain (text below image)

And I dont't know a clean way to combine this with a multipart/alternative HTML-email.

来源：https://stackoverflow.com/questions/10631856/mime-type-to-satisfy-html-email-images-and-plain-text

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