Added error message if user attempts to POST to multiple/add view without AJAX

wagtail/wagtailimages/views/multiple.py

@@ -16,10 +16,13 @@ def add(request):
     Image = get_image_model()
     ImageForm = get_image_form_for_multi()
 
-    if request.POST and request.is_ajax():
+    if request.POST:
+        if not request.is_ajax():
+            return HttpResponseBadRequest("Cannot POST to this view without AJAX")
+
         if not request.FILES:
             return HttpResponseBadRequest("Must upload a file")
-        else:
+
         image = Image(uploaded_by_user=request.user,  title=request.FILES['files[]'].name, file=request.FILES['files[]'])
         image.save()
         form = ImageForm(instance=image, prefix='image-%d'%image.id)