view_mail_template.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="view_mail_template_form_inherit" model="ir.ui.view">
            <field name="name">mail.template.inherit</field>
            <field name="model">mail.template</field>
            <field name="inherit_id" ref="mail.email_template_form"/>
            <field name="arch" type="xml">
                <xpath expr="//notebook" position="inside">
                    <page string="Custom Attachments">
                        <group>
                            <field name="mail_attachment_custom_ids" nolabel="1"/>
                        </group>
                    </page>
                </xpath>
            </field>
        </record>
    </data>
</openerp>