be explicite in parent folder type when checking for uniqueness (#4628)

src/collective/dms/mailcontent/dmsmail.py

@@ -36,7 +36,7 @@ def validateIndexValueUniqueness(context, portal_type, index_name, value):
         return
     catalog = getToolByName(context, 'portal_catalog')
     brains = catalog.searchResults(**{index_name: value})
-    if context.portal_type != portal_type:
+    if context.portal_type in ('Folder', 'pfwbgedfolder'):
         # we create the dmsincomingmail, the context is the container
         if brains:
             raise Invalid(_(u"This value is already used"))