Index: wp-admin/js/widgets/media-widgets.js
===================================================================
--- wp-admin/js/widgets/media-widgets.js	(revision 40833)
+++ wp-admin/js/widgets/media-widgets.js	(working copy)
@@ -255,7 +255,7 @@
 				}
 			});
 			if ( specificMimes.length > 0 ) {
-				mime = specificMimes.join( ',' );
+				mime = specificMimes;
 			}
 
 			this.states.add([