How can I modify the class for a button in a django widget form?

I'm trying to set the class for a form widget. I have:

<form action="{% url "list" %}" method="post" enctype="multipart/form-data">
        {% csrf_token %}
        <p>{{ form.non_field_errors }}</p>
        <p>{{ form.input.label_tag }} {{ form.input.help_text }}</p>
        <p>
            {{ form.input.errors }}
            {{ form.input }}
        </p>
        <p>{{ form.non_field_errors }}</p>
        <p>{{ form.itinerary.label_tag }} {{ form.itinerary.help_text }}</p>
        <p>
            {{ form.itinerary.errors }}
            {{ form.itinerary }}
        </p>
        <p><input class="button-primary" type="submit" value="Upload" /></p>
</form>

and it looks like

I attempted to use django tweak tools when I tried googling to find a solution for my problem. I modified the code to be like this too see how it would change:

<form action="{% url "list" %}" method="post" enctype="multipart/form-data">
        {% csrf_token %}
        {% load widget_tweaks %}
        <p>{{ form.non_field_errors }}</p>
        <p>{{ form.input.label_tag }} {{ form.input.help_text }}</p>
        <p>
            {{ form.input.errors }}
            {{ form.input }}
            {% render_field form.input class="button" %}
        </p>
        <p>{{ form.non_field_errors }}</p>
        <p>{{ form.itinerary.label_tag }} {{ form.itinerary.help_text }}</p>
        <p>
            {{ form.itinerary.errors }}
            {{ form.itinerary }}
        </p>
        <p><input class="button-primary" type="submit" value="Upload" /></p>
    </form>

and that looks like

It applies the class to the whole widget, and it turns the whole widget into a button.

So how can I apply the class to just the button in the widget?