mail-addons/mail_all/views/templates.xml

<?xml version="1.0"?>
<!--# Copyright 2016 Ivan Yelizariev <https://it-projects.info/team/yelizariev>
    # Copyright 2018 Artyom Losev <https://it-projects.info/team/ArtyomLosev>
    # Copyright 2018 Kolushov Alexandr <https://it-projects.info/team/KolushovAlexandr>
    # License LGPL-3.0 or later (https://www.gnu.org/licenses/lgpl.html). -->
<odoo>
        <template id="mail_all_assets_backend"
                  name="mail_all_assets_backend"
                  inherit_id="web.assets_backend">
            <xpath expr="." position="inside">
                <link rel="stylesheet" href="/mail_all/static/src/css/mail_all.css"/>
                <script src="/mail_all/static/src/js/mail_all.js" type="text/javascript"></script>
                <script src="/mail_all/static/src/js/test_mail_all.js" type="text/javascript"></script>
            </xpath>
        </template>
</odoo>