gnus-summary-save-parts-default-mime

gnus-summary-save-parts-default-mime is a variable defined in `gnus-sum.el'.
Its value is

"image/.*"

Documentation:
*A regexp to match MIME parts when saving multiple parts of a
message with `gnus-summary-save-parts' (X m).
This regexp will be used by default when prompting the user for which
type of files to save.

You can customize this variable.