social/mail_attach_existing_attachment/wizard/mail_compose_message_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data noupdate="0">
        <record model="ir.ui.view" id="email_compose_message_wizard_inherit_form">
            <field name="name">mail.compose.message.form (mail_attach_existing_attachment)</field>
            <field name="model">mail.compose.message</field>
            <field name="inherit_id" ref="mail.email_compose_message_wizard_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='attachment_ids']" position="after">
                    <field name="can_attach_attachment" invisible="1"/>
                    <div attrs="{'invisible': [('can_attach_attachment', '=', False)]}">
                        <br />
                        <field name="object_attachment_ids" widget="many2many_checkboxes" domain="[('res_model', '=', model), ('res_id', '=', res_id)]" />
                    </div>
                </xpath>
            </field>
        </record>
    </data>
</openerp>