social/mail_attach_existing_attachment_account/wizard/account_invoice_send_view.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
    Copyright 2019 Thore Baden
    License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
-->
<odoo>
    <record id="account_invoice_send_wizard_form" model="ir.ui.view">
        <field name="name">Send Invoice</field>
        <field name="model">account.invoice.send</field>
        <field name="inherit_id" ref="account.account_invoice_send_wizard_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='attachment_ids']" position="after">
                <field name="model" invisible="1"/>
                <field name="res_id" invisible="1"/>
                <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>
</odoo>