diff --git a/plone/app/discussion/comment.py b/plone/app/discussion/comment.py
index 9d9b1e1..df81bf7 100644
--- a/plone/app/discussion/comment.py
+++ b/plone/app/discussion/comment.py
@@ -93,7 +93,7 @@ class Comment(CatalogAware, WorkflowAware, DynamicType, Traversable,
 
     title = u""
 
-    mime_type = "text/plain"
+    mime_type = None
     text = u""
 
     creator = None
@@ -113,6 +113,7 @@ class Comment(CatalogAware, WorkflowAware, DynamicType, Traversable,
 
     def __init__(self):
         self.creation_date = self.modification_date = datetime.utcnow()
+        self.mime_type = 'text/plain'
 
     @property
     def __name__(self):