Author: David Maus Date: 2010-03-25 21:56:50 CET Table of Contents ================= 1 Representing a MIME internet message 2 MIME delimiters of SEMI and mml 3 Generic function 4 Open questions 4.1 How to handle charset information? 4.2 How to attach files? 5 Quotes from the specs 5.1 MIME multipart: Notion of structured, related body parts 5.2 MIME multipart: order of entities inside a multipart 1 Representing a MIME internet message ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A MIME internet message consists of one or more MIME entities. Each MIME entity is of a distinct type and subtype, has a body and optional MIME headers related to it's content. A MIME entity is represented as a list: (TYPE SUBTYPE BODY CONT-HEAD) TYPE: Symbol of MIME media type (e.g. text, video, audio). SUBTYPE: Symbol of MIME media subtype (e.g. plain, html). BODY: String with entity body -or- list of other MIME entities. CONT-HEAD: List of cons with content related MIME header fields. The name of the header field without the prefix "Content-" is car, the value cdr. Example: '(text html "

Headline

" ((disposition . inline))) For messages of type multipart the body consists of a list of one or more MIME entities. '(multipart alternative '((text plain "* Headline") (text html "

headline

"))) 2 MIME delimiters of SEMI and mml ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The MIME delimiters are defined in an association list with a symbol of the library's name as key and delimiter format strings as values. For each library there are three formatstrings. (SYMBOL DELIM-SINGLE DELIM-SINGLE-CONT DELIM-MULTI) DELIM-SINGLE: Denoting a single MIME entity. Strings are passed in this order: 1. type 2. subtype 3. content header 4. body DELIM-SINGLE-CONT: Format of content header strings. Strings are passed in this order: 1. header field name 2. header field value DELIM-MULTI: Enclosing parts of a multipart entity. Strings are passed in this order: 1. subtype 2. body 3. subtype (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>"))) 3 Generic function ~~~~~~~~~~~~~~~~~~~ This generic function returns a string representation with MIME delimiters depending on the variable =org-mail-htmlize-mime-lib=. (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)))) 4 Open questions ~~~~~~~~~~~~~~~~~ 4.1 How to handle charset information? ======================================= 4.2 How to attach files? ========================= 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. 5 Quotes from the specs ~~~~~~~~~~~~~~~~~~~~~~~~ 5.1 MIME multipart: Notion of structured, related body parts ============================================================= - [RFC2046, 5.1.1] ORG-BLOCKQUOTE-START 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. ORG-BLOCKQUOTE-END [RFC2046, 5.1.1]: http://tools.ietf.org/html/rfc2046.html#section-5.1.1 5.2 MIME multipart: order of entities inside a multipart ========================================================= - [RFC2046, 5.1.3] ORG-BLOCKQUOTE-START 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". ORG-BLOCKQUOTE-END - [RFC2046, 5.1.4] ORG-BLOCKQUOTE-START 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. ORG-BLOCKQUOTE-END ORG-BLOCKQUOTE-START 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. ORG-BLOCKQUOTE-END [RFC2046, 5.1.3]: http://tools.ietf.org/html/rfc2046.html#section-5.1.3 [RFC2046, 5.1.4]: http://tools.ietf.org/html/rfc2046.html#section-5.1.4