code: def _send_from_directories(directories, filename, **options):
      
      for directory in directories:
          filename = safe_join(directory, filename)
          if not os.path.isabs(filename):
              filename = os.path.join(current_app.root_path, filename)
          try:
              if not os.path.isfile(filename):
                  continue
          except (TypeError, ValueError):
              raise BadRequest()
          options.setdefault('conditional', True)
          return send_file(filename, **options)
      raise NotFound()
docs: Send a file from a given directory with :func:`send_file`.  This
      is a secure way to quickly expose static files from an upload folder
      or something similar.
  
      Example usage::
  
          @app.route('/uploads/<path:filename>')
          def download_file(filename):
              return send_from_directory(app.config.UPLOAD_FOLDER,
                                         filename, as_attachment=True)
  
      .. admonition:: Sending files and Performance
  
         It is strongly recommended to activate either ``X-Sendfile`` support in
         your webserver or (if no authentication happens) to tell the webserver
         to serve files for the given path on its own without calling into the
         web application for improved performance.
  
      :param directories: the list of directories to look for filename.
      :param filename: the filename relative to that directory to
                       download.
      :param options: optional keyword arguments that are directly
                      forwarded to :func:`send_file`.