diff --git a/addons/mail/tests/test_mail_template.py b/addons/mail/tests/test_mail_template.py
index 1d618e956e9..34e6e57e1e2 100644
--- a/addons/mail/tests/test_mail_template.py
+++ b/addons/mail/tests/test_mail_template.py
@@ -72,7 +72,7 @@ class TestMailTemplate(TestMail):
         static attachments are not duplicated and while reports are re-generated,
         and that intermediary attachments are dropped."""
 
-        composer = self.env['mail.compose.message'].create({})
+        composer = self.env['mail.compose.message'].with_context(default_attachment_ids=[]).create({})
         report_template = self.env.ref('web.action_report_externalpreview')
         template_1 = self.email_template.copy({
             'report_template': report_template.id,
@@ -82,20 +82,20 @@ class TestMailTemplate(TestMail):
             'report_template': report_template.id,
         })
 
-        onchange_templates = [template_1, template_2, template_1]
-        attachments_onchange = []
+        onchange_templates = [template_1, template_2, template_1, False]
+        attachments_onchange = [composer.attachment_ids]
         # template_1 has two static attachments and one dynamically generated report,
         # template_2 only has the report, so we should get 3, 1, 3 attachments
-        attachment_numbers = [3, 1, 3]
+        attachment_numbers = [0, 3, 1, 3, 0]
 
-        for template in onchange_templates:
-            onchange = composer.onchange_template_id(
-                template.id, 'comment', 'mail.test', self.test_pigs.id
-            )
-            values = composer._convert_to_record(composer._convert_to_cache(onchange['value']))
-            attachments = values['attachment_ids']
-            composer.attachment_ids = attachments  # we apply the onchange
-            attachments_onchange.append(attachments)
+        with self.env.do_in_onchange():
+            for template in onchange_templates:
+                onchange = composer.onchange_template_id(
+                    template.id if template else False, 'comment', 'mail.test', self.test_pigs.id
+                )
+                values = composer._convert_to_record(composer._convert_to_cache(onchange['value']))
+                attachments_onchange.append(values['attachment_ids'])
+                composer.update(onchange['value'])
 
         self.assertEqual(
             [len(attachments) for attachments in attachments_onchange],
@@ -103,7 +103,7 @@ class TestMailTemplate(TestMail):
         )
 
         self.assertTrue(
-            len(attachments_onchange[0] & attachments_onchange[2]) == 2,
+            len(attachments_onchange[1] & attachments_onchange[3]) == 2,
             "The two static attachments on the template should be common to the two onchanges"
         )
 
diff --git a/addons/mail/wizard/mail_compose_message.py b/addons/mail/wizard/mail_compose_message.py
index 7516f507590..7347e7ad7d5 100644
--- a/addons/mail/wizard/mail_compose_message.py
+++ b/addons/mail/wizard/mail_compose_message.py
@@ -350,7 +350,6 @@ class MailComposer(models.TransientModel):
             - normal mode: return rendered values
             /!\ for x2many field, this onchange return command instead of ids
         """
-        attachment_ids = []
         if template_id and composition_mode == 'mass_mail':
             template = self.env['mail.template'].browse(template_id)
             fields = ['subject', 'body_html', 'email_from', 'reply_to', 'mail_server_id']
@@ -366,6 +365,7 @@ class MailComposer(models.TransientModel):
             values = self.generate_email_for_composer(template_id, [res_id])[res_id]
             # transform attachments into attachment_ids; not attached to the document because this will
             # be done further in the posting process, allowing to clean database if email not send
+            attachment_ids = []
             Attachment = self.env['ir.attachment']
             for attach_fname, attach_datas in values.pop('attachments', []):
                 data_attach = {
@@ -377,6 +377,8 @@ class MailComposer(models.TransientModel):
                     'type': 'binary',  # override default_type from context, possibly meant for another model!
                 }
                 attachment_ids.append(Attachment.create(data_attach).id)
+            if values.get('attachment_ids', []) or attachment_ids:
+                values['attachment_ids'] = [(5,)] + values.get('attachment_ids', []) + attachment_ids
         else:
             default_values = self.with_context(default_composition_mode=composition_mode, default_model=model, default_res_id=res_id).default_get(['composition_mode', 'model', 'res_id', 'parent_id', 'partner_ids', 'subject', 'body', 'email_from', 'reply_to', 'attachment_ids', 'mail_server_id'])
             values = dict((key, default_values[key]) for key in ['subject', 'body', 'partner_ids', 'email_from', 'reply_to', 'attachment_ids', 'mail_server_id'] if key in default_values)
@@ -388,10 +390,7 @@ class MailComposer(models.TransientModel):
         # ORM handle the assignation of command list on new onchange (api.v8),
         # this force the complete replacement of x2many field with
         # command and is compatible with onchange api.v7
-        attachment_ids += values.pop('attachment_ids' , [])
         values = self._convert_to_write(values)
-        if attachment_ids:
-            values.update(attachment_ids=[(6, 0, attachment_ids)])
 
         return {'value': values}