mail-addons/mail_attachment_popup/views/mail_attachment_popup_template.xml

<?xml version="1.0" encoding="utf-8" ?>
<openerp>
    <data>
        <template
            id="assets_backend"
            name="mail attachment popup assets"
            inherit_id="web.assets_backend"
        >
            <xpath expr="." position="inside">
                <link
                    rel="stylesheet"
                    href="/mail_attachment_popup/static/src/css/jquery.arcticmodal.css"
                />
                <link
                    rel="stylesheet"
                    href="/mail_attachment_popup/static/src/css/simple.css"
                />
                <link
                    rel="stylesheet"
                    href="/mail_attachment_popup/static/src/css/styles.css"
                />
                <script
                    type="text/javascript"
                    src="/mail_attachment_popup/static/lib/js/jquery.arcticmodal.js"
                />
            </xpath>
        </template>
    </data>
</openerp>