Save as dialog not showing

Question

I'd like to show a save as dialog for a file download through a HTTPHandler (GET) in a Google App Engine application (Webapp2).

Although I am using the attachment content disposition like:

   self.response.headers["Content-Type"] = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
   self.response.headers["Content-Disposition"] = "attachment; filename='test.xls'"

The file is automatically downloaded and the save-as dialog not shown