mml-content-disposition-alist

mml-content-disposition-alist is a variable defined in `mml.el'.
Its value is

((text
  (rtf . "attachment")
  (t . "inline"))
 (t . "attachment"))

Documentation:
Alist of MIME types or regexps matching file names and default dispositions.
Each element should be one of the following three forms:

(REGEXP . DISPOSITION)
(SUPERTYPE (SUBTYPE . DISPOSITION) (SUBTYPE . DISPOSITION)...)
(TYPE . DISPOSITION)

Where REGEXP is a string which matches the file name (if any) of an
attachment, SUPERTYPE, SUBTYPE and TYPE should be symbols which are a
MIME supertype (e.g., text), a MIME subtype (e.g., plain) and a MIME
type (e.g., text/plain) respectively, and DISPOSITION should be either
the string "attachment" or the string "inline". The value t for
SUPERTYPE, SUBTYPE or TYPE matches any of those types. The first
match found will be used.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.