Fix form css class context key

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/sources/views.py

@@ -192,7 +192,6 @@ class UploadBaseView(MultiFormView):
 
 
 class UploadInteractiveView(UploadBaseView):
-
     def dispatch(self, request, *args, **kwargs):
         self.subtemplates_list = []
 
@@ -354,7 +353,7 @@ class UploadInteractiveView(UploadBaseView):
                             kwargs=self.request.resolver_match.kwargs
                         ), self.request.META['QUERY_STRING']
                     ),
-                    'form_class': 'dropzone',
+                    'form_css_classes': 'dropzone',
                     'form_disable_submit': True,
                     'form_id': 'html5upload',
                 }