docs: def allowed_extension(*allowed):
      '''
      refs: http://zhuoqiang.me/a/restful-pyramid
      Custom predict checking if the the file extension
      of the request URI is in the allowed set.
      '''
      def predicate(info, request):
          log.debug(request.path)
          ext = os.path.splitext(request.path)[1]
          request.environ['path_extenstion'] = ext
          log.debug(ext)
          return ext in allowed
  
      return predicate