code: def serve(self, filename):
          '''Serve a file given its filename'''
          if not self.exists(filename):
              abort(404)
          return self.backend.serve(filenamef serve(self, filename):
          '''Serve a file given its filename'''
          if not self.exists(filename):
              abort(404)
          return self.backend.serve(filename)
docs: Serve a file given its filename