mail-addons/mail_base/views/templates.xml

<?xml version="1.0" ?>
<openerp>
    <data>
        <template
            id="mail_base_assets_backend"
            name="mail_base_assets_backend"
            inherit_id="web.assets_backend"
        >
            <xpath expr="." position="inside">
                <script src="/mail_base/static/lib/base.js" type="text/javascript" />
            </xpath>
        </template>
    </data>
</openerp>