GeezerVison

'(text html "<h1>Headline</h1>" ((disposition . inline)))

'(multipart alternative
            '((text plain "* Headline")
              (text html "<h1>headline</h1>")))

(setq org-mail-htmlize-mime-delimiter-alist
      '((semi "\n--[[%s/%s%s]]\n%s" "\ncontent-%s: %s" "\n--<<%s>>-{\n%s\n--}-<<%s>>")
        (mml "\n<#part type=\"%s/%s\"%s>\n%s" " %s=\"%s\"" "\n<#multipart type=\"%s\">\n%s\n<#/multipart>")))

(setq org-mail-htmlize-mime-lib 'semi)

(defun org-mail-htmlize-mime-entity (type subtype body
                                          &optional cont-head)
  "Return string representation of MIME entity.

TYPE is the type of entity body.
SUBTYPE is the subtype of body.
BODY is the body of the entity.  Either a string with the body
content or a list with one ore more MIME entities.
Optional argument CONT-HEAD is a list of cons with content
specific headers, name in car and value in cdr."
  (let ((delimlst (assoc org-mail-htmlize-mime-lib
                         org-mail-htmlize-mime-delimiter-alist)))
    (if (eq type 'multipart)
        (format (nth 3 delimlst) subtype
                (mapconcat (lambda (b)
                              (apply 'org-mail-htmlize-mime-entity
                                     (car b) (cadr b) (cddr b)))
                           body "")
                subtype)
      (format (nth 1 delimlst)
              type subtype
              (mapconcat (lambda (h)
                            (format (nth 2 delimlst) (car h) (cdr h)))
                         cont-head "")
              body))))

The generic function expects BODY either be a string or a list. Attaching binary file (image, etc.) requires to encode it so the message will pass the message system. So we might use kind of a encoder (e.g. base64) on our own.

Or, what seems a cleaner solution: Use attachment function of the respective MIME mode. To achive this: Introduce third possibility for BODY: A cons with the filename in car and symbol of the function in cdr.

(FILENAME . FUNCTION)

'(image jpeg ("/path/to/image" . org-mail-htmlize-add-attachment))

The function org-mail-htmlize-add-attachment is called with file name as argument and calls the appropriate function depending on org-mail-htmlize-mime-lib and returns a string

• with the encoded body

  -or-

• the complete MIME entity

Side effect: The user might be prompted for attachment settings (e.g. encoding). But, on the other hand: We delegate the job of creating the attachment to the library that is responsible for mime.

• RFC2046, 5.1.1

  NOTE: Conspicuously missing from the “multipart” type is a notion of structured, related body parts. It is recommended that those wishing to provide more structured or integrated multipart messaging facilities should define subtypes of multipart that are syntactically identical but define relationships between the various parts. For example, subtypes of multipart could be defined that include a distinguished part which in turn is used to specify the relationships between the other parts, probably referring to them by their Content-ID field. Old implementations will not recognize the new subtype if this approach is used, but will treat it as multipart/mixed and will thus be able to show the user the parts that are recognized.

• RFC2046, 5.1.3

  5.1.3. Mixed Subtype

  The “mixed” subtype of “multipart” is intended for use when the body parts are independent and need to be bundled in a particular order. Any “multipart” subtypes that an implementation does not recognize must be treated as being of subtype “mixed”.

• RFC2046, 5.1.4

  5.1.4. Alternative Subtype

  The “multipart/alternative” type is syntactically identical to “multipart/mixed”, but the semantics are different. In particular, each of the body parts is an “alternative” version of the same information.

  Systems should recognize that the content of the various parts are interchangeable. Systems should choose the “best” type based on the local environment and references, in some cases even through user interaction. As with “multipart/mixed”, the order of body parts is significant. In this case, the alternatives appear in an order of increasing faithfulness to the original content. In general, the best choice is the LAST part of a type supported by the recipient system’s local environment.

  In general, user agents that compose “multipart/alternative” entities must place the body parts in increasing order of preference, that is, with the preferred format last. For fancy text, the sending user agent should put the plainest format first and the richest format last. Receiving user agents should pick and display the last format they are capable of displaying. In the case where one of the alternatives is itself of type “multipart” and contains unrecognized sub-parts, the user agent may choose either to show that alternative, an earlier alternative, or both.