diff --git a/plone/app/discussion/browser/comments.py b/plone/app/discussion/browser/comments.py
index 1eddf0a..6be93a2 100644
--- a/plone/app/discussion/browser/comments.py
+++ b/plone/app/discussion/browser/comments.py
@@ -168,6 +168,10 @@ class CommentForm(extensible.ExtensibleForm, form.Form):
 
         # Create comment
         comment = createObject('plone.Comment')
+
+        # Set comment mime type to current setting in the discussion registry
+        comment.mime_type = settings.text_transform
+
         # Set comment attributes (including extended comment form attributes)
         for attribute in self.fields.keys():
             setattr(comment, attribute, data[attribute])
diff --git a/plone/app/discussion/comment.py b/plone/app/discussion/comment.py
index 78baf8b..7b4c12c 100644
--- a/plone/app/discussion/comment.py
+++ b/plone/app/discussion/comment.py
@@ -90,7 +90,7 @@ class Comment(CatalogAware, WorkflowAware, DynamicType, Traversable,
 
     title = u""
 
-    mime_type = None
+    mime_type = "text/plain"
     text = u""
 
     creator = None
@@ -205,6 +205,10 @@ def notify_content_object(obj, event):
     """Tell the content object when a comment is added
     """
     content_obj = aq_parent(aq_parent(obj))
+    # set the modified date and reindex the item accordingly
+    # so that 304s work correctly. This means that adding a comment
+    # effectively counts as modifying the content type.
+    content_obj.setModificationDate()
     content_obj.reindexObject(idxs=('total_comments',
                                     'last_comment_date',
                                     'commentators',
diff --git a/plone/app/discussion/tests/test_comment.py b/plone/app/discussion/tests/test_comment.py
index 307c646..3afe87e 100644
--- a/plone/app/discussion/tests/test_comment.py
+++ b/plone/app/discussion/tests/test_comment.py
@@ -127,6 +127,10 @@ class CommentTest(unittest.TestCase):
         comment1 = createObject('plone.Comment')
         self.assertEqual(comment1.Type(), 'Comment')
 
+    def test_mime_type(self):
+        comment1 = createObject('plone.Comment')
+        self.assertEqual(comment1.mime_type, 'text/plain')
+
     def test_getText(self):
         comment1 = createObject('plone.Comment')
         comment1.text = """First paragraph