web/web_group_expand/templates/assets.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <template id="assets_backend" name="web_groupby_expand assets" inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <link rel="stylesheet" href="/web_group_expand/static/src/less/web_group_expand.less"/>
            <script type="text/javascript" src="/web_group_expand/static/src/js/web_group_expand.js"></script>
            <script type="text/javascript" src="/web_group_expand/static/src/js/web_group_expand_menu.js"></script>
        </xpath>
    </template>
</odoo>