myaac/system/templates/admin.plugins.form.html.twig

<div class="row">
    <div class="col-md-3">
        <div class="box box-primary collapsed-box">
            <div class="box-header with-border">
                <h3 class="box-title">Install plugin:</h3>
                <div class="box-tools pull-right">
                    <button type="button" class="btn btn-box-tool" data-widget="collapse"><i class="fa fa-plus"></i>
                    </button>
                </div>
            </div>
            <div class="box-body">
                <form enctype="multipart/form-data" method="post" action="{{ constant('ADMIN_URL') }}?p=plugins">
                    <input type="hidden" name="upload_plugin"/>
                    <div class="box-body">
                        <div class="form-group">
                            <label for="exampleInputFile">File input</label>
                            <input type="file" name="plugin">
                        </div>
                    </div>
                    <div class="box-footer">
                        <button type="submit" class="btn btn-primary">Upload</button>
                    </div>
                </form>
            </div>
        </div>
    </div>
</div>