{% extends 'SyliusWebBundle:Backend:layout.html.twig' %}

{% block content %}
<div class="page-header">
    <h1>{{ 'sylius.prototype.build_header'|trans|raw }}</h1>
</div>

<form action="{{ path('sylius_backend_prototype_build', {'id': prototype.id}) }}" method="post" class="form-horizontal" novalidate>
    {% include 'SyliusWebBundle:Backend/Product:_form.html.twig' %}
    <div class="form-actions">
        <input type="submit" value="{{ 'sylius.create'|trans }}" class="btn btn-primary btn-large" />
        <a href="{{ path('sylius_backend_prototype_index') }}" class="btn btn-large">{{ 'sylius.cancel'|trans }}</a>
    </div>
</form>
{% endblock %}

{% extends 'SyliusWebBundle:Backend:layout.html.twig' %}

{% block content %}
<div class="page-header">
    <h1>{{ 'sylius.prototype.build_header'|trans|raw }}</h1>
</div>

<form action="{{ path('sylius_backend_prototype_build', {'id': prototype.id}) }}" method="post" class="form-horizontal" {{ form_enctype(form) }} novalidate>
    {% include 'SyliusWebBundle:Backend/Product:_form.html.twig' %}
    <div class="form-actions">
        <input type="submit" value="{{ 'sylius.create'|trans }}" class="btn btn-primary btn-large" />
        <a href="{{ path('sylius_backend_prototype_index') }}" class="btn btn-large">{{ 'sylius.cancel'|trans }}</a>
    </div>
</form>
{% endblock %}

Use form enctype in prototype build template
Use form enctype in prototype build template