-(defvar gnus-mime-display-multipart-as-mixed nil)
-(defvar gnus-mime-display-multipart-alternative-as-mixed nil)
-(defvar gnus-mime-display-multipart-related-as-mixed nil)
+(defcustom gnus-mime-display-multipart-as-mixed nil
+ "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'."
+ :group 'gnus-article-mime
+ :type 'boolean)
+
+(defcustom gnus-mime-display-multipart-alternative-as-mixed nil
+ "Display \"multipart/alternative\" parts as \"multipart/mixed\"."
+ :group 'gnus-article-mime
+ :type 'boolean)
+
+(defcustom gnus-mime-display-multipart-related-as-mixed nil
+ "Display \"multipart/related\" parts as \"multipart/mixed\".
+
+If displaying \"text/html\" is discouraged \(see
+`mm-discouraged-alternatives'\) images or other material inside a
+\"multipart/related\" part might be overlooked when this variable is `nil'."
+ :group 'gnus-article-mime
+ :type 'boolean)