gnus-mime-display-multipart-as-mixed

gnus-mime-display-multipart-as-mixed is a variable defined in `gnus-art.el'.
Its value is

nil

Documentation:
Display "multipart" parts as "multipart/mixed".

If t, it overrides nil values of
`gnus-mime-display-multipart-alternative-as-mixed' and
`gnus-mime-display-multipart-related-as-mixed'.

You can customize this variable.