convert into string only if a result is returned

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@41126 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Tool/ContributionTool.py

@@ -630,6 +630,8 @@ class ContributionTool(BaseTool):
       return
     portal = self.getPortalObject()
     content_type = portal.mimetypes_registry.lookupExtension(filename)
+    if content_type:
+      return str(content_type)
     return content_type
 
   def _openURL(self, url):