template: use url_for() to create download file url

templates/download.html

@@ -5,7 +5,7 @@
 {% block head %}
     {{ super() }}
     {% if filename is defined %}
-        <meta http-equiv="refresh" content="0;URL='download_file/{{ filename }}'" />
+    <meta http-equiv="refresh" content="0;URL='{{ url_for('download_file', filename=filename) }}'" />
     {% endif %}
     <link href="//cdnjs.cloudflare.com/ajax/libs/bootstrap-datetimepicker/4.17.47/css/bootstrap-datetimepicker.min.css" rel="stylesheet">
 {% endblock %}