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4.4 MIME Support

The Multipurpose Internet Mail Extensions (MIME) define a standard means for structuring mail messages. MIME permits a message to have multiple parts, each of which is called an entity. It also provides a way to associate type information with each entity. For example, an ordinary text message has type `text/plain', HTML has type `text/html', and a JPEG image has type `image/jpeg'. Additionally, MIME entities may be annotated to indicate whether they should be shown in-line, or whether they are attachments that should be shown only upon further user action.

IMAIL provides simple support for MIME messages. MIME attachments are shown in the IMAIL buffer by special abbreviations. You can write an attachment to a file. Multipart MIME structures are recognized and displayed in a clean format that suppresses unnecessary clutter. And MIME encodings such as quoted-printable and base64 are automatically decoded prior to displaying the message or saving the attachment.

End-user formatting of MIME messages is a complex process, partly because these messages can be arbitrarily complex in their internal structure. IMAIL provides several variables that give you some control over the formatting prcess.

Many MIME messages have multiple parts; for example, a message with an attachment normally contains at least two parts: the message text and the attachment. IMAIL separates the different parts of a MIME message with specially-formatted lines. There are several styles of separator lines available, selected by changing the value of the variable imail-mime-boundary-style. The default value of simple means to use long lines of hyphen characters as the separator lines. A value of sgml means use long lines of hyphens that are wrapped with `<!--' and `-->', which makes them valid SGML comments. A value of original means to use the original MIME boundaries, which have certain useful syntactic properties but are not as visually distinctive.

MIME also specifies a particular kind of multipart message, of type `multipart/alternative', in which the parts are different representations of the same message. A typical example of this is a mailer that sends both plain text and HTML versions of the message text. Normally IMAIL shows only the simplest of these parts (which is almost always plain text) and suppresses the alternatives. However, if you set the variable imail-mime-show-alternatives to #t, IMAIL will show these alternative forms as attachments.

Another kind of multipart MIME message is the digest message, which has type `multipart/digest'. Digest messages are normally used by high-volume mailing lists to reduce the number of messages sent to the end user; instead the user receives one message containing all of the messages from that list in a particular time period, usually a day. IMAIL can present MIME digest messages in one of two formats. The default format is to show all of the component messages of the digest as attachments. This is particularly useful for large digests that you will only read a few messages from, since you can scan the digest contents for interesting messages without downloading all of the messages in the digest. In the alternative format, selected by setting imail-mime-collapse-digest to #f, the component messages of a digest are all shown inline.

As a general rule, any MIME entity that contains non-textual information is displayed as an attachment. Attachments are normally shown as specially-formatted abbreviations. Here is an example:


This shows various things about the attachment, including its (optional) name, its MIME type, and the length of the attachment in bytes. (The length is computed on the encoded form of the attachment, and is generally slightly larger than the decoded length.)

IMAIL uses somewhat more complicated rules for deciding when a MIME entity is displayed in this abbreviated format, and when it is expanded in line. In general, all non-text entities are abbreviated. Additionally, if a text entity is given a MIME disposition of `attachment', if the character set of the entity is unknown, if the encoding type is unknown, or if the subtype is unknown, it is abbreviated.

Two variables control the abbreviation of text entities. imail-known-mime-charsets is a list of regular expressions that specify the known character sets; by default it specifies US-ASCII, the ISO 8859 character sets, and some random but commonly-seen Microsoft Windows character sets. The variable imail-inline-mime-text-subtypes contains a list of symbols, each of which is the name of a text subtype that should be shown in line. For example, if the symbol html is in this list, then MIME parts of type text/html are shown in-line. Text subtypes not appearing in this list are abbreviated as attachments.

Here are IMAIL's MIME-specific commands:

Save a MIME attachment to a file (imail-save-attachment).
Save an arbitrary MIME entity (message part) to a file (imail-save-mime-entity).
C-c C-t C-e
Toggle a MIME entity between its formatted and raw forms (imail-toggle-mime-entity).

The primary MIME command is C-o (imail-save-attachment), which saves a single attachment to a file. If point is on an attachment, that is the attachment to be saved, otherwise IMAIL prompts for an attachment by name. If a prefix argument is specified, prompting is performed even if point is on an attachment. Once the attachment is determined, IMAIL prompts for the name of a file to save the attachment to. The filename is initialized from the name specified by the attachment, if any. The directory of the filename is initialized to the directory in which the last attachment was saved, or the user's home directory if no attachments have previously been saved.

If you want to save attachments to a specific directory, change the variable imail-mime-attachment-directory to contain the name of that directory.

The command w (imail-save-mime-entity) is similar to imail-save-attachment except that it will save any MIME entity, not just an attachment. For example, this allows you to save the message text. This command saves the entity that point is on; if point is not on any entity, an error is signalled. If the entity is encoded, e.g. with quoted-printable or base64 encoding, it is decoded before it is saved. If the entity is text, it is written to the file in text mode (relevant only under Windows and OS/2); otherwise it is written in binary mode.

A simpler way to save a MIME entity is to point at the entity with the mouse and click the right button (imail-mouse-save-mime-entity). This works the same way as imail-save-mime-entity except that the entity is selected by the mouse instead of point.

The command C-c C-t C-e (imail-toggle-mime-entity) is similar to imail-save-mime-entity, except that instead of saving the entity to a file, it toggles whether the entity is shown in-line or in abbreviated form. A common situation in which this is useful is when the text of a message is in an unknown character set. In this case, IMAIL by default shows the text in abbreviated form; use C-t to expand it in place.