<form action={{url}} method="POST" enctype="multipart/form-data">
  {% for name, value in params.items %}
    <input type="hidden" name="{{name}}" value="{{value}}"/>
  {% endfor %}
  {% block extra %} {% endblock %}
  {% block file %} 
    <input type="file" name="file"/>
  {% endblock %}
  {% block submit %} 
    <input type="submit"/>
  {% endblock %}
</form>
