mail_tracking/views/assets.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- © 2016 Antonio Espinosa - <antonio.espinosa@tecnativa.com>
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html). -->
<openerp>
<data>

<template id="assets_backend"
          name="mail_tracking assets"
          inherit_id="web.assets_backend">
    <xpath expr="." position="inside">
        <link rel="stylesheet"
              href="/mail_tracking/static/src/css/mail_tracking.css"/>
        <script type="text/javascript"
                src="/mail_tracking/static/src/js/mail_tracking.js"/>
    </xpath>
</template>

</data>
</openerp>