format_number/views/header.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <template id="format_number.assets_backend" inherit_id="web.assets_backend">
            <xpath expr="." position="inside">
                <script type="text/javascript" src="/format_number/static/src/js/number_widget.js"/>
            </xpath>
        </template>
    </data>
</openerp>