The Mutt E-Mail Client: Mutt's MIME Support

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## 5. Mutt's MIME Support

Quite a bit of effort has been made to make Mutt the premier text-mode MIME MUA. Every effort has been made to provide the functionality that the discerning MIME user requires, and the conformance to the standards wherever possible. When configuring Mutt for MIME, there are two extra types of configuration files which Mutt uses. One is the `mime.types` file, which contains the mapping of file extensions to IANA MIME types. The other is the `mailcap` file, which specifies the external commands to use for handling specific MIME types.

## 5.1 Using MIME in Mutt

There are three areas/menus in Mutt which deal with MIME, they are the pager (while viewing a message), the attachment menu and the compose menu.

### Viewing MIME messages in the pager

When you select a message from the index and view it in the pager, Mutt decodes the message to a text representation. Mutt internally supports a number of MIME types, including `text/plain, text/enriched, message/rfc822, and message/news`. In addition, the export controlled version of Mutt recognizes a variety of PGP MIME types, including PGP/MIME and application/pgp.

Mutt will denote attachments with a couple lines describing them. These lines are of the form:

> `

>     [-- Attachment #1: Description --]
>     [-- Type: text/plain, Encoding: 7bit, Size: 10000 --]

> `

Where the `Description` is the description or filename given for the attachment, and the `Encoding` is one of `7bit/8bit/quoted-printable/base64/binary`.

If Mutt cannot deal with a MIME type, it will display a message like:

> `

>     [-- image/gif is unsupported (use 'v' to view this part) --]

> `

### The Attachment Menu

The default binding for `view-attachments` is `v', which displays the attachment menu for a message. The attachment menu displays a list of the attachments in a message. From the attachment menu, you can save, print, pipe, delete, and view attachments. You can apply these operations to a group of attachments at once, by tagging the attachments and by using the ``tag-prefix'' operator. You can also reply to the current message from this menu, and only the current attachment (or the attachments tagged) will be quoted in your reply. You can view attachments as text, or view them using the mailcap viewer definition.

Finally, you can apply the usual message-related functions (like resend-message, and the reply and forward functions) to attachments of type `message/rfc822`.

See the help on the attachment menu for more information.

### The Compose Menu

The compose menu is the menu you see before you send a message. It allows you to edit the recipient list, the subject, and other aspects of your message. It also contains a list of the attachments of your message, including the main body. From this menu, you can print, copy, filter, pipe, edit, compose, review, and rename an attachment or a list of tagged attachments. You can also modifying the attachment information, notably the type, encoding and description.

Attachments appear as follows:

    -   1 [text/plain, 7bit, 1K]             /tmp/mutt-euler-8082-0 <no description>
        2 [applica/x-gunzip, base64, 422K]   ~/src/mutt-0.85.tar.gz <no description>

The '-' denotes that Mutt will delete the file after sending (or postponing, or cancelling) the message. It can be toggled with the `toggle-unlink` command (default: u). The next field is the MIME content-type, and can be changed with the `edit-type` command (default: ^T). The next field is the encoding for the attachment, which allows a binary message to be encoded for transmission on 7bit links. It can be changed with the `edit-encoding` command (default: ^E). The next field is the size of the attachment, rounded to kilobytes or megabytes. The next field is the filename, which can be changed with the `rename-file` command (default: R). The final field is the description of the attachment, and can be changed with the `edit-description` command (default: d).

## 5.2 MIME Type configuration with `mime.types`

When you add an attachment to your mail message, Mutt searches your personal mime.types file at `${HOME}/.mime.types`, and then the system mime.types file at `/usr/local/share/mutt/mime.types` or `/etc/mime.types`

The mime.types file consist of lines containing a MIME type and a space separated list of extensions. For example:

> `

>     application/postscript          ps eps
>     application/pgp                 pgp
>     audio/x-aiff                    aif aifc aiff

> `

A sample `mime.types` file comes with the Mutt distribution, and should contain most of the MIME types you are likely to use.

If Mutt can not determine the mime type by the extension of the file you attach, it will look at the file. If the file is free of binary information, Mutt will assume that the file is plain text, and mark it as `text/plain`. If the file contains binary information, then Mutt will mark it as `application/octet-stream`. You can change the MIME type that Mutt assigns to an attachment by using the `edit-type` command from the compose menu (default: ^T). The MIME type is actually a major mime type followed by the sub-type, separated by a '/'. 6 major types: application, text, image, video, audio, and model have been approved after various internet discussions. Mutt recognises all of these if the appropriate entry is found in the mime.types file. It also recognises other major mime types, such as the chemical type that is widely used in the molecular modelling community to pass molecular data in various forms to various molecular viewers. Non-recognised mime types should only be used if the recipient of the message is likely to be expecting such attachments.

## 5.3 MIME Viewer configuration with `mailcap`

Mutt supports RFC 1524 MIME Configuration, in particular the Unix specific format specified in Appendix A of RFC 1524. This file format is commonly referred to as the mailcap format. Many MIME compliant programs utilize the mailcap format, allowing you to specify handling for all MIME types in one place for all programs. Programs known to use this format include Netscape, XMosaic, lynx and metamail.

In order to handle various MIME types that Mutt can not handle internally, Mutt parses a series of external configuration files to find an external handler. The default search string for these files is a colon delimited list set to

> `

>     ${HOME}/.mailcap:/usr/local/share/mutt/mailcap:/etc/mailcap:/etc/mailcap:/usr/etc/mailcap:/usr/local/etc/mailcap

> `

where `$HOME` is your home directory.

In particular, the metamail distribution will install a mailcap file, usually as `/usr/local/etc/mailcap`, which contains some baseline entries.

### The Basics of the mailcap file

A mailcap file consists of a series of lines which are comments, blank, or definitions.

A comment line consists of a # character followed by anything you want.

A blank line is blank.

A definition line consists of a content type, a view command, and any number of optional fields. Each field of a definition line is divided by a semicolon ';' character.

The content type is specified in the MIME standard type/subtype method. For example, `text/plain, text/html, image/gif,` etc. In addition, the mailcap format includes two formats for wildcards, one using the special '*' subtype, the other is the implicit wild, where you only include the major type. For example, `image/*`, or `video,` will match all image types and video types, respectively.

The view command is a Unix command for viewing the type specified. There are two different types of commands supported. The default is to send the body of the MIME message to the command on stdin. You can change this behaviour by using %s as a parameter to your view command. This will cause Mutt to save the body of the MIME message to a temporary file, and then call the view command with the %s replaced by the name of the temporary file. In both cases, Mutt will turn over the terminal to the view program until the program quits, at which time Mutt will remove the temporary file if it exists.

So, in the simplest form, you can send a text/plain message to the external pager more on stdin:

> `

>     text/plain; more

> `

Or, you could send the message as a file:

> `

>     text/plain; more %s

> `

Perhaps you would like to use lynx to interactively view a text/html message:

> `

>     text/html; lynx %s

> `

In this case, lynx does not support viewing a file from stdin, so you must use the %s syntax. Note: Some older versions of lynx contain a bug where they will check the mailcap file for a viewer for text/html. They will find the line which calls lynx, and run it. This causes lynx to continuously spawn itself to view the object.

On the other hand, maybe you don't want to use lynx interactively, you just want to have it convert the text/html to text/plain, then you can use:

> `

>     text/html; lynx -dump %s | more

> `

Perhaps you wish to use lynx to view text/html files, and a pager on all other text formats, then you would use the following:

> `

>     text/html; lynx %s
>     text/*; more

> `

This is the simplest form of a mailcap file.

### Secure use of mailcap

The interpretion of shell meta-characters embedded in MIME parameters can lead to security problems in general. Mutt tries to quote parameters in expansion of %s syntaxes properly, and avoids risky characters by substituting them, see the mailcap_sanitize variable.

Although mutt's procedures to invoke programs with mailcap seem to be safe, there are other applications parsing mailcap, maybe taking less care of it. Therefore you should pay attention to the following rules:

Keep the %-expandos away from shell quoting. Don't quote them with single or double quotes. Mutt does this for you, the right way, as should any other program which interprets mailcap. Don't put them into backtick expansions. Be highly careful with eval statements, and avoid them if possible at all. Trying to fix broken behaviour with quotes introduces new leaks - there is no alternative to correct quoting in the first place.

If you have to use the %-expandos' values in context where you need quoting or backtick expansions, put that value into a shell variable and reference the shell variable where necessary, as in the following example (using `$charset` inside the backtick expansion is safe, since it is not itself subject to any further expansion):

> `

>     text/test-mailcap-bug; cat %s; copiousoutput; test=charset=%{charset} \
>             && test "`echo $charset | tr '[A-Z]' '[a-z]'`" != iso-8859-1

> `

### Advanced mailcap Usage

### Optional Fields

In addition to the required content-type and view command fields, you can add semi-colon ';' separated fields to set flags and other options. Mutt recognizes the following optional fields:

copiousoutput

  This flag tells Mutt that the command passes possibly large amounts of text on stdout. This causes Mutt to invoke a pager (either the internal pager or the external pager defined by the pager variable) on the output of the view command. Without this flag, Mutt assumes that the command is interactive. One could use this to replace the pipe to `more` in the `lynx -dump` example in the Basic section:

  > `

  >     text/html; lynx -dump %s ; copiousoutput

  > `

  This will cause lynx to format the text/html output as text/plain and Mutt will use your standard pager to display the results.
needsterminal

  Mutt uses this flag when viewing attachments with autoview, in order to decide whether it should honor the setting of the $wait_key variable or not. When an attachment is viewed using an interactive program, and the corresponding mailcap entry has a needsterminal flag, Mutt will use $wait_key and the exit status of the program to decide if it will ask you to press a key after the external program has exited. In all other situations it will not prompt you for a key.

compose=<command>

  This flag specifies the command to use to create a new attachment of a specific MIME type. Mutt supports this from the compose menu.

composetyped=<command>

  This flag specifies the command to use to create a new attachment of a specific MIME type. This command differs from the compose command in that mutt will expect standard MIME headers on the data. This can be used to specify parameters, filename, description, etc. for a new attachment. Mutt supports this from the compose menu.

print=<command>

  This flag specifies the command to use to print a specific MIME type. Mutt supports this from the attachment and compose menus.

edit=<command>

  This flag specifies the command to use to edit a specific MIME type. Mutt supports this from the compose menu, and also uses it to compose new attachments. Mutt will default to the defined editor for text attachments.

nametemplate=<template>

  This field specifies the format for the file denoted by %s in the command fields. Certain programs will require a certain file extension, for instance, to correctly view a file. For instance, lynx will only interpret a file as `text/html` if the file ends in `.html`. So, you would specify lynx as a `text/html` viewer with a line in the mailcap file like:

  > `

  >     text/html; lynx %s; nametemplate=%s.html

  > `

test=<command>

  This field specifies a command to run to test whether this mailcap entry should be used. The command is defined with the command expansion rules defined in the next section. If the command returns 0, then the test passed, and Mutt uses this entry. If the command returns non-zero, then the test failed, and Mutt continues searching for the right entry. Note: the content-type must match before Mutt performs the test. For example:

  > `

  >     text/html; netscape -remote 'openURL(%s)' ; test=RunningX
  >     text/html; lynx %s

  > `

  In this example, Mutt will run the program RunningX which will return 0 if the X Window manager is running, and non-zero if it isn't. If RunningX returns 0, then Mutt will call netscape to display the text/html object. If RunningX doesn't return 0, then Mutt will go on to the next entry and use lynx to display the text/html object.

### Search Order

When searching for an entry in the mailcap file, Mutt will search for the most useful entry for its purpose. For instance, if you are attempting to print an `image/gif`, and you have the following entries in your mailcap file, Mutt will search for an entry with the print command:

> `

>     image/*;        xv %s
>     image/gif;      ; print= anytopnm %s | pnmtops | lpr; \
>                     nametemplate=%s.gif

> `

Mutt will skip the `image/*` entry and use the `image/gif` entry with the print command.

In addition, you can use this with Autoview to denote two commands for viewing an attachment, one to be viewed automatically, the other to be viewed interactively from the attachment menu. In addition, you can then use the test feature to determine which viewer to use interactively depending on your environment.

> `

>     text/html;      netscape -remote 'openURL(%s)' ; test=RunningX
>     text/html;      lynx %s; nametemplate=%s.html
>     text/html;      lynx -dump %s; nametemplate=%s.html; copiousoutput

> `

For Autoview, Mutt will choose the third entry because of the copiousoutput tag. For interactive viewing, Mutt will run the program RunningX to determine if it should use the first entry. If the program returns non-zero, Mutt will use the second entry for interactive viewing.

### Command Expansion

The various commands defined in the mailcap files are passed to the `/bin/sh` shell using the system() function. Before the command is passed to `/bin/sh -c`, it is parsed to expand various special parameters with information from Mutt. The keywords Mutt expands are:

%s

  As seen in the basic mailcap section, this variable is expanded to a filename specified by the calling program. This file contains the body of the message to view/print/edit or where the composing program should place the results of composition. In addition, the use of this keyword causes Mutt to not pass the body of the message to the view/print/edit program on stdin.

%t

  Mutt will expand %t to the text representation of the content type of the message in the same form as the first parameter of the mailcap definition line, ie `text/html` or `image/gif`.

%{<parameter>}

  Mutt will expand this to the value of the specified parameter from the Content-Type: line of the mail message. For instance, if Your mail message contains:

  > `

  >     Content-Type: text/plain; charset=iso-8859-1

  > `

  then Mutt will expand %{charset} to iso-8859-1. The default metamail mailcap file uses this feature to test the charset to spawn an xterm using the right charset to view the message.
\%

  This will be replaced by a %

Mutt does not currently support the %F and %n keywords specified in RFC 1524. The main purpose of these parameters is for multipart messages, which is handled internally by Mutt.

### Example mailcap files

This mailcap file is fairly simple and standard:

    # I'm always running X :)
    video/*;        xanim %s > /dev/null
    image/*;        xv %s > /dev/null

    # I'm always running netscape (if my computer had more memory, maybe)
    text/html;      netscape -remote 'openURL(%s)'

This mailcap file shows quite a number of examples:

    # Use xanim to view all videos   Xanim produces a header on startup,
    # send that to /dev/null so I don't see it
    video/*;        xanim %s > /dev/null

    # Send html to a running netscape by remote
    text/html;      netscape -remote 'openURL(%s)'; test=RunningNetscape

    # If I'm not running netscape but I am running X, start netscape on the
    # object
    text/html;      netscape %s; test=RunningX

    # Else use lynx to view it as text
    text/html;      lynx %s

    # This version would convert the text/html to text/plain
    text/html;      lynx -dump %s; copiousoutput

    # enriched.sh converts text/enriched to text/html and then uses
    # lynx -dump to convert it to text/plain
    text/enriched;  enriched.sh ; copiousoutput

    # I use enscript to print text in two columns to a page
    text/*;         more %s; print=enscript -2Gr %s

    # Netscape adds a flag to tell itself to view jpegs internally
    image/jpeg;xv %s; x-mozilla-flags=internal

    # Use xv to view images if I'm running X
    # In addition, this uses the \ to extend the line and set my editor
    # for images
    image/*;xv %s; test=RunningX; \
            edit=xpaint %s

    # Convert images to text using the netpbm tools
    image/*;  (anytopnm %s | pnmscale -xysize 80 46 | ppmtopgm | pgmtopbm |
    pbmtoascii -1x2 ) 2>&1 ; copiousoutput

    # Send excel spreadsheets to my NT box
    application/ms-excel; open.pl %s

## 5.4 MIME Autoview

In addition to explicitly telling Mutt to view an attachment with the MIME viewer defined in the mailcap file, Mutt has support for automatically viewing MIME attachments while in the pager.

To work, you must define a viewer in the mailcap file which uses the `copiousoutput` option to denote that it is non-interactive. Usually, you also use the entry to convert the attachment to a text representation which you can view in the pager.

You then use the `auto_view` muttrc command to list the content-types that you wish to view automatically.

For instance, if you set auto_view to:

> `

>     auto_view text/html text/enriched application/x-gunzip application/postscript image/gif application/x-tar-gz

> `

Mutt could use the following mailcap entries to automatically view attachments of these types.

> `

>     text/html;      lynx -dump %s; copiousoutput; nametemplate=%s.html
>     text/enriched;  enriched.sh  ; copiousoutput
>     image/*;        anytopnm %s | pnmscale -xsize 80 -ysize 50 | ppmtopgm | pgmtopbm | pbmtoascii ; copiousoutput
>     application/x-gunzip;   gzcat; copiousoutput
>     application/x-tar-gz; gunzip -c %s | tar -tf - ; copiousoutput
>     application/postscript; ps2ascii %s; copiousoutput

> `

## 5.5 MIME Multipart/Alternative

Mutt has some heuristics for determining which attachment of a multipart/alternative type to display. First, mutt will check the alternative_order list to determine if one of the available types is preferred. The alternative_order list consists of a number of mimetypes in order, including support for implicit and explicit wildcards, for example:

> `

>     alternative_order text/enriched text/plain text application/postscript image/*

> `

Next, mutt will check if any of the types have a defined auto_view, and use that. Failing that, Mutt will look for any text type. As a last attempt, mutt will look for any type it knows how to handle.

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