gnus-article-mime-match-handle-function

gnus-article-mime-match-handle-function is a variable defined in `gnus-art.el'.
Its value is

undisplayed-alternative

• This variable may be risky if used as a file-local variable.

Documentation:
Function called with a MIME handle as the argument.
This is meant for people who want to view first matched part.
For `undisplayed-alternative' (default), the first undisplayed
part or alternative part is used. For `undisplayed', the first
undisplayed part is used. For a function, the first part which
the function return t is used. For nil, the first part is
used.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.