diff --git a/addons/mail/models/ir_attachment.py b/addons/mail/models/ir_attachment.py
index 92e9e29804d..3661ef336cc 100644
--- a/addons/mail/models/ir_attachment.py
+++ b/addons/mail/models/ir_attachment.py
@@ -58,6 +58,9 @@ class IrAttachment(models.Model):
                 'name': attachment.name,
                 'mimetype': 'application/octet-stream' if safari and attachment.mimetype and 'video' in attachment.mimetype else attachment.mimetype,
             }
+            if attachment.res_id and issubclass(self.pool[attachment.res_model], self.pool['mail.thread']):
+                main_attachment = self.env[attachment.res_model].sudo().browse(attachment.res_id).message_main_attachment_id
+                res['is_main'] = attachment == main_attachment
             if commands:
                 res['originThread'] = [('insert', {
                     'id': attachment.res_id,
diff --git a/addons/mail/models/mail_message.py b/addons/mail/models/mail_message.py
index daca9e6e019..a0403666a97 100644
--- a/addons/mail/models/mail_message.py
+++ b/addons/mail/models/mail_message.py
@@ -845,12 +845,7 @@ class Message(models.Model):
                 author = (0, message_sudo.email_from)
 
             # Attachments
-            main_attachment = self.env['ir.attachment']
-            if message_sudo.attachment_ids and message_sudo.res_id and issubclass(self.pool[message_sudo.model], self.pool['mail.thread']):
-                main_attachment = self.env[message_sudo.model].sudo().browse(message_sudo.res_id).message_main_attachment_id
             attachments_formatted = message_sudo.attachment_ids._attachment_format()
-            for attachment in attachments_formatted:
-                attachment['is_main'] = attachment['id'] == main_attachment.id
             # Tracking values
             tracking_value_ids = []
             for tracking in message_sudo.tracking_value_ids: