mail_attachtment_custom_data.xml

<?xml version="1.0"?>
<odoo>
    <data noupdate="1">
        <record id="mail_attachment_custom_ftop" model="mail.attachment.custom">
            <field name="name">Fichiers joints pour les membres FTOP</field>
            <field name="mail_template_id" ref="coop_membership.welcome_email"/>
            <field name="condition">${object.shift_type == 'ftop'}</field>
        </record>

        <record id="mail_attachment_custom_standard" model="mail.attachment.custom">
            <field name="name">Fichiers joints pour les membres Standard</field>
            <field name="mail_template_id" ref="coop_membership.welcome_email"/>
            <field name="condition">${object.shift_type == 'standard'}</field>
        </record>
    </data>
</odoo>