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1. Decoding and Viewing

This chapter deals with decoding and viewing MIME messages on a higher level.

The main idea is to first analyze a MIME article, and then allow other programs to do things based on the list of handles that are returned as a result of this analysis.

1.1 Dissection  Analyzing a MIME message.
1.2 Non-MIME  Analyzing a non-MIME message.
1.3 Handles  Handle manipulations.
1.4 Display  Displaying handles.
1.5 Display Customization  Variables that affect display.
1.6 Files and Directories  Saving and naming attachments.
1.7 New Viewers  How to write your own viewers.

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1.1 Dissection

The mm-dissect-buffer is the function responsible for dissecting a MIME article. If given a multipart message, it will recursively descend the message, following the structure, and return a tree of MIME handles that describes the structure of the message.

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1.2 Non-MIME

Gnus also understands some non-MIME attachments, such as postscript, uuencode, binhex, yenc, shar, forward, gnatsweb, pgp, diff. Each of these features can be disabled by add an item into mm-uu-configure-list. For example,

(require 'mm-uu)
(add-to-list 'mm-uu-configure-list '(pgp-signed . disabled))

Postscript file.

Uuencoded file.

Binhex encoded file.

Yenc encoded file.

Shar archive file.

Non-MIME forwarded message.

Gnatsweb attachment.

PGP signed clear text.

PGP encrypted clear text.

PGP public keys.

Emacs source code. This item works only in the groups matching mm-uu-emacs-sources-regexp.

Patches. This is intended for groups where diffs of committed files are automatically sent to. It only works in groups matching mm-uu-diff-groups-regexp.

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1.3 Handles

A MIME handle is a list that fully describes a MIME component.

The following macros can be used to access elements in a handle:

Return the buffer that holds the contents of the undecoded MIME part.

Return the parsed Content-Type of the part.

Return the Content-Transfer-Encoding of the part.

Return the object that can be used to remove the displayed part (if it has been displayed).

Set the undisplayer object.

Return the parsed Content-Disposition of the part.

Returns the handle(s) referred to by Content-ID.

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1.4 Display

Functions for displaying, removing and saving.

Display the part.

Remove the part (if it has been displayed).

Say whether a MIME type can be displayed inline.

Say whether a MIME type should be displayed automatically.

Free all resources occupied by a part.

Offer to save the part in a file.

Offer to pipe the part to some process.

Prompt for a mailcap method to use to view the part.

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1.5 Display Customization

This is an alist where the key is a MIME type, the second element is a function to display the part inline (i.e., inside Emacs), and the third element is a form to be evaled to say whether the part can be displayed inline.

This variable specifies whether a part can be displayed inline, and, if so, how to do it. It does not say whether parts are actually displayed inline.

This, on the other hand, says what types are to be displayed inline, if they satisfy the conditions set by the variable above. It's a list of MIME media types.

This is a list of types that are to be displayed "automatically", but only if the above variable allows it. That is, only inlinable parts can be displayed automatically.

This is a list of types that will be displayed automatically in an external viewer.

This is a list of media types for which the external viewer will not be killed when selecting a different article.

Some MIME agents create parts that have a content-disposition of `attachment'. This variable allows overriding that disposition and displaying the part inline. (Note that the disposition is only overridden if we are able to, and want to, display the part inline.)

List of MIME types that are discouraged when viewing `multipart/alternative'. Viewing agents are supposed to view the last possible part of a message, as that is supposed to be the richest. However, users may prefer other types instead, and this list says what types are most unwanted. If, for instance, `text/html' parts are very unwanted, and `text/richtext' parts are somewhat unwanted, you could say something like:

(setq mm-discouraged-alternatives
      '("text/html" "text/richtext")
      (remove "text/html" mm-automatic-display))

Adding "image/.*" might also be useful. Spammers use images as the prefered part of `multipart/alternative' messages, so you might not notice there are other parts. See also gnus-buttonized-mime-types, section `MIME Commands' in Gnus Manual. After adding "multipart/alternative" to gnus-buttonized-mime-types you can choose manually which alternative you'd like to view. For example, you can set those variables like:

(setq gnus-buttonized-mime-types
      '("multipart/alternative" "multipart/signed")
      '("text/html" "image/.*"))

In this case, Gnus will display radio buttons for such a kind of spam message as follows:

1.  (*) multipart/alternative  ( ) image/gif

2.  (*) text/plain          ( ) text/html

When displaying inline images that are larger than the window, Emacs does not enable scrolling, which means that you cannot see the whole image. To prevent this, the library tries to determine the image size before displaying it inline, and if it doesn't fit the window, the library will display it externally (e.g. with `ImageMagick' or `xv'). Setting this variable to t disables this check and makes the library display all inline images as inline, regardless of their size.

mm-inlined-types may include regular expressions, for example to specify that all `text/.*' parts be displayed inline. If a user prefers to have a type that matches such a regular expression be treated as an attachment, that can be accomplished by setting this variable to a list containing that type. For example assuming mm-inlined-types includes `text/.*', then including `text/html' in this variable will cause `text/html' parts to be treated as attachments.

This selects the function used to render HTML. The predefined renderers are selected by the symbols w3, w3m(1), links, lynx, w3m-standalone or html2text. If nil use an external viewer. You can also specify a function, which will be called with a MIME handle as the argument.

Some HTML mails might have the trick of spammers using `<img>' tags. It is likely to be intended to verify whether you have read the mail. You can prevent your personal informations from leaking by setting this option to nil (which is the default). It is currently ignored by Emacs/w3. For emacs-w3m, you may use the command t on the image anchor to show an image even if it is nil.(2)

A regular expression that matches safe URL names, i.e. URLs that are unlikely to leak personal information when rendering HTML email (the default value is `\\`cid:'). If nil consider all URLs safe.

You can use emacs-w3m command keys in the inlined text/html part by setting this option to non-nil. The default value is t.

The program used to start an external terminal.

Indicate whether external MIME handlers should be used.

If t, all defined external MIME handlers are used. If nil, files are saved to disk (mailcap-save-binary-file). If it is the symbol ask, you are prompted before the external MIME handler is invoked.

When you launch an attachment through mailcap (see section 4.12 mailcap) an attempt is made to use a safe viewer with the safest options--this isn't the case if you save it to disk and launch it in a different way (command line or double-clicking). Anyhow, if you want to be sure not to launch any external programs, set this variable to nil or ask.

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1.6 Files and Directories

The default directory for saving attachments. If nil use default-directory.

Directory for storing temporary files.

A list of functions used for rewriting file names of MIME parts. Each function is applied successively to the file name. Ready-made functions include

Delete all control characters.

Delete characters that could have unintended consequences when used with flawed shell scripts, i.e. `|', `>' and `<'; and `-', `.' as the first character.

Remove all whitespace.

Remove leading and trailing whitespace.

Collapse multiple whitespace characters.

Replace whitespace with underscores. Set the variable mm-file-name-replace-whitespace to any other string if you do not like underscores.

The standard Emacs functions capitalize, downcase, upcase and upcase-initials might also prove useful.

List of functions used for rewriting the full file names of MIME parts. This is used when viewing parts externally, and is meant for transforming the absolute name so that non-compliant programs can find the file where it's saved.

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1.7 New Viewers

Here's an example viewer for displaying text/enriched inline:

(defun mm-display-enriched-inline (handle)
  (let (text)
      (mm-insert-part handle)
        (enriched-decode (point-min) (point-max))
        (setq text (buffer-string))))
    (mm-insert-inline handle text)))

We see that the function takes a MIME handle as its parameter. It then goes to a temporary buffer, inserts the text of the part, does some work on the text, stores the result, goes back to the buffer it was called from and inserts the result.

The two important helper functions here are mm-insert-part and mm-insert-inline. The first function inserts the text of the handle in the current buffer. It handles charset and/or content transfer decoding. The second function just inserts whatever text you tell it to insert, but it also sets things up so that the text can be "undisplayed" in a convenient manner.

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