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new.html.twig

{% extends '::base.html.twig' %}
{% block body %}
    {{ form(form) }}

    <script>
        $(document).ready(function(){
            var $file = $('#book_file');
            $file.parent().after(
                    '<div class="form-group">' +
                    '<label class="control-label" for="covers[]">Cover(s)</label>' +
                    '<input type="file" name="covers[]" />'
            );
        });
    </script>
{% endblock %}