Attach a file to the outgoing MIME message.
The file is not inserted or encoded until you send the message with
`M-x message-send-and-exit' or `M-x message-send' in Message mode,
or `M-x mail-send-and-exit' or `M-x mail-send' in Mail mode.
FILE is the name of the file to attach. TYPE is its
content-type, a string of the form "type/subtype". DESCRIPTION
is a one-line description of the attachment. The DISPOSITION
specifies how the attachment is intended to be displayed. It can
be either "inline" (displayed automatically within the message
body) or "attachment" (separate from the body).
(fn FILE &optional TYPE DESCRIPTION DISPOSITION)