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 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 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 canceling) 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).

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 behavior 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.

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.

The interpretation 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 behavior 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

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 firefox (if my computer had more memory, maybe)
text/html;      firefox -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 firefox by remote
text/html;      firefox -remote 'openURL(%s)'; test=RunningFirefox

# If I'm not running firefox but I am running X, start firefox on the
# object
text/html;      firefox %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

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

# Firefox 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