diff --git a/addons/account/models/account_move.py b/addons/account/models/account_move.py
index db90f984acd..494ffe94c79 100644
--- a/addons/account/models/account_move.py
+++ b/addons/account/models/account_move.py
@@ -49,7 +49,7 @@ EMPTY = object()
 
 class AccountMove(models.Model):
     _name = "account.move"
-    _inherit = ['portal.mixin', 'mail.thread', 'mail.activity.mixin', 'sequence.mixin']
+    _inherit = ['portal.mixin', 'mail.thread.main.attachment', 'mail.activity.mixin', 'sequence.mixin']
     _description = "Journal Entry"
     _order = 'date desc, name desc, id desc'
     _mail_post_access = 'read'
diff --git a/addons/account/models/account_payment.py b/addons/account/models/account_payment.py
index 20eb353866c..c8634590845 100644
--- a/addons/account/models/account_payment.py
+++ b/addons/account/models/account_payment.py
@@ -7,7 +7,7 @@ from odoo.tools.misc import format_date, formatLang
 class AccountPayment(models.Model):
     _name = "account.payment"
     _inherits = {'account.move': 'move_id'}
-    _inherit = ['mail.thread', 'mail.activity.mixin']
+    _inherit = ['mail.thread.main.attachment', 'mail.activity.mixin']
     _description = "Payments"
     _order = "date desc, name desc"
     _check_company_auto = True
diff --git a/addons/crm/data/crm_lead_demo.xml b/addons/crm/data/crm_lead_demo.xml
index 8cbc3c03a4a..31385b1349d 100644
--- a/addons/crm/data/crm_lead_demo.xml
+++ b/addons/crm/data/crm_lead_demo.xml
@@ -444,10 +444,6 @@ Andrew</p>]]></field>
             <field name="subtype_id" ref="mail.mt_comment"/>
             <field name="attachment_ids" eval="[(6, 0, [ref('msg_case15_attach1')])]"/>
         </record>
-        <!--Main attachment is usually registered by message_post, so here we must set it manually-->
-        <record id="crm_case_15" model="crm.lead">
-            <field name="message_main_attachment_id" ref="msg_case15_attach1"/>
-        </record>
         <record id="msg_case15_3" model="mail.message">
             <field name="subject">Re: Plan to buy RedHat servers</field>
             <field name="model">crm.lead</field>
diff --git a/addons/hr/models/hr_employee.py b/addons/hr/models/hr_employee.py
index a851e18d937..81c807f801a 100644
--- a/addons/hr/models/hr_employee.py
+++ b/addons/hr/models/hr_employee.py
@@ -26,7 +26,7 @@ class HrEmployeePrivate(models.Model):
     _name = "hr.employee"
     _description = "Employee"
     _order = 'name'
-    _inherit = ['hr.employee.base', 'mail.thread', 'mail.activity.mixin', 'resource.mixin', 'avatar.mixin']
+    _inherit = ['hr.employee.base', 'mail.thread.main.attachment', 'mail.activity.mixin', 'resource.mixin', 'avatar.mixin']
     _mail_post_access = 'read'
 
     # resource and user
diff --git a/addons/hr_expense/models/hr_expense.py b/addons/hr_expense/models/hr_expense.py
index 2910a4db5cc..bdb4654c6a5 100644
--- a/addons/hr_expense/models/hr_expense.py
+++ b/addons/hr_expense/models/hr_expense.py
@@ -15,7 +15,7 @@ from odoo.tools import email_split, float_repr, float_compare, is_html_empty
 class HrExpense(models.Model):
 
     _name = "hr.expense"
-    _inherit = ['mail.thread', 'mail.activity.mixin', 'analytic.mixin']
+    _inherit = ['mail.thread.main.attachment', 'mail.activity.mixin', 'analytic.mixin']
     _description = "Expense"
     _order = "date desc, id desc"
     _check_company_auto = True
@@ -784,7 +784,7 @@ class HrExpenseSheet(models.Model):
         =================================================================================
     """
     _name = "hr.expense.sheet"
-    _inherit = ['mail.thread', 'mail.activity.mixin']
+    _inherit = ['mail.thread.main.attachment', 'mail.activity.mixin']
     _description = "Expense Report"
     _order = "accounting_date desc, id desc"
     _check_company_auto = True
diff --git a/addons/hr_holidays/models/hr_leave.py b/addons/hr_holidays/models/hr_leave.py
index a03bc41a9f7..9612ed30749 100644
--- a/addons/hr_holidays/models/hr_leave.py
+++ b/addons/hr_holidays/models/hr_leave.py
@@ -72,7 +72,7 @@ class HolidaysRequest(models.Model):
     _name = "hr.leave"
     _description = "Time Off"
     _order = "date_from desc"
-    _inherit = ['mail.thread', 'mail.activity.mixin']
+    _inherit = ['mail.thread.main.attachment', 'mail.activity.mixin']
     _mail_post_access = 'read'
 
     @api.model
diff --git a/addons/hr_recruitment/models/hr_applicant.py b/addons/hr_recruitment/models/hr_applicant.py
index 0a091639ba7..e9730b74a46 100644
--- a/addons/hr_recruitment/models/hr_applicant.py
+++ b/addons/hr_recruitment/models/hr_applicant.py
@@ -22,7 +22,7 @@ class Applicant(models.Model):
     _name = "hr.applicant"
     _description = "Applicant"
     _order = "priority desc, id desc"
-    _inherit = ['mail.thread.cc', 'mail.activity.mixin', 'utm.mixin']
+    _inherit = ['mail.thread.cc', 'mail.thread.main.attachment', 'mail.activity.mixin', 'utm.mixin']
     _mailing_enabled = True
     _primary_email = 'email_from'