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.
You can also create arbitrarily complex multiparts using the MML language (see Composing in The Emacs MIME Manual).