Flow doesn't handle accept=undefined on the incoming options correctly.

@@ -186,9 +186,11 @@ define(function(require) {
                     var eventHandler = Upload.flowEventHandlers[eventName] || makeFallbackFlowEventHandler(eventName);
                     flow.on(eventName, _.bind(eventHandler, instance));
                 });
-                flow.assignBrowse(el, true, false, {
-                    accept: model.get('accept'),
-                });
+                var assignOptions = {};
+                if (locals.accept) {
+                    assignOptions.accept = locals.accept;
+                }
+                flow.assignBrowse(el, true, false, assignOptions);
                 flow.assignDrop(el);
                 instance.view = rivets.bind($(el).contents(), this.view.models);
                 return {};