erp5_base: guessMimeType: set only if necessary

bt5/erp5_base/WorkflowTemplateItem/portal_workflow/document_conversion_interaction_workflow/scripts/Document_guessMimeType.py

@@ -4,5 +4,8 @@ portal = document.getPortalObject()
 filename = document.getFilename()
 content_type = portal.mimetypes_registry.lookupExtension(filename)
 
-if content_type is not None:
-  document.setContentType(str(content_type))
+if content_type is None:
+  return
+content_type = str(content_type)
+if content_type != document.getContentType():
+  document.setContentType(content_type)