https://bitbucket.org/bbangert/webhelpers/commits/9ad434bec9a16c06c1cfeed38cde02f00a95685d
diff -ur WebHelpers-1.3.orig/webhelpers/mimehelper.py WebHelpers-1.3/webhelpers/mimehelper.py
--- webhelpers/mimehelper.py	2010-11-18 12:34:33.000000000 +0800
+++ webhelpers/mimehelper.py	2013-06-17 15:45:40.603257640 +0800
@@ -110,8 +110,7 @@
             if '.' in last_part:
                 has_extension = True
         if 'HTTP_ACCEPT' in self.env:
-            possible_from_accept_header = webob.acceptparse.MIMEAccept('ACCEPT', 
-                self.env['HTTP_ACCEPT'])
+            possible_from_accept_header = webob.acceptparse.MIMEAccept(self.env['HTTP_ACCEPT'])
         if has_extension == False:
             if possible_from_accept_header is None:
                 return self._set_response_content_type(content_type)