Message is a MIME-compliant posting agent. The user generally
doesn’t have to do anything to make the MIME happen—Message will
automatically add the Content-Type and
Content-Transfer-Encoding headers.
The most typical thing users want to use the multipart things in MIME for is to add “attachments” to mail they send out. This can be done with the C-c C-a command (M-x mml-attach-file), which will prompt for a file name and a MIME type.
If your Emacs supports drag and drop, you can also drop the file in the
Message buffer. The variable mml-dnd-protocol-alist specifies
what kind of action is done when you drop a file into the Message
buffer. The variable mml-dnd-attach-options controls which
MIME options you want to specify when dropping a file. If it
is a list, valid members are type, description and
disposition. disposition implies type. If it is
nil, don’t ask for options. If it is t, ask the user
whether or not to specify options.
If your system supports it, you can also insert screenshots directly
into the Message buffer. The C-c C-p
(message-insert-screenshot) command inserts the image into the
buffer as an MML part, and puts an image text property on
top. The message-screenshot-command variable says what
external command to use to take the screenshot. It defaults to
"import png:-", which is an ImageMagick command.
You can also create arbitrarily complex multiparts using the MML language (see Composing in The Emacs MIME Manual).