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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 preferred part of ‘multipart/alternative’ messages, so you might not notice there are other parts. See also gnus-buttonized-mime-types, MIME Commands. 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. If you set this variable to resize, the image will be displayed resized to fit in the window, if Emacs has the ability to resize images.
The proportion used when resizing large images.
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 gnus-article-html, w3m1, 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.
If this is non-nil, inhibit displaying of images inline in the article body. It is effective to images in HTML articles rendered when mm-text-html-renderer (see Display Customization) is shr or w3m. In Gnus, this is overridden by the value of gnus-inhibit-images (see Misc Article). The default is nil.
External images that have URLs that match this regexp won't be fetched and displayed. For instance, to block all URLs that have the string “ads” in them, do the following:
          (setq mm-html-blocked-images "ads")

It is effective when mm-text-html-renderer (see Display Customization) is shr. In Gnus, this is overridden by the value of gnus-blocked-images or the return value of the function that gnus-blocked-images is set to (see HTML).

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 information from leaking by setting this option to "" (which is the default).

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. In Gnus, this will be overridden according to the value of the variable gnus-safe-html-newsgroups, See Various Various.
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 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.


[1] See for more information about emacs-w3m