diff --git a/addons/im_livechat/models/mail_message.py b/addons/im_livechat/models/mail_message.py
index d6e08bc511e..998289ef39b 100644
--- a/addons/im_livechat/models/mail_message.py
+++ b/addons/im_livechat/models/mail_message.py
@@ -7,7 +7,7 @@ from odoo import models
 class MailMessage(models.Model):
     _inherit = 'mail.message'
 
-    def _message_format(self, fnames, format_reply=True, legacy=False):
+    def _message_format(self, fnames, format_reply=True):
         """Override to remove email_from and to return the livechat username if applicable.
         A third param is added to the author_id tuple in this case to be able to differentiate it
         from the normal name in client code.
@@ -17,7 +17,7 @@ class MailMessage(models.Model):
         This allows the frontend display to include the additional features
         (e.g: Show additional buttons with the available answers for this step). """
 
-        vals_list = super()._message_format(fnames=fnames, format_reply=format_reply, legacy=legacy)
+        vals_list = super()._message_format(fnames=fnames, format_reply=format_reply)
         for vals in vals_list:
             message_sudo = self.browse(vals['id']).sudo().with_prefetch(self.ids)
             discuss_channel = self.env['discuss.channel'].browse(message_sudo.res_id) if message_sudo.model == 'discuss.channel' else self.env['discuss.channel']
diff --git a/addons/mail/models/ir_attachment.py b/addons/mail/models/ir_attachment.py
index 8d9b2855d97..707b7383a0d 100644
--- a/addons/mail/models/ir_attachment.py
+++ b/addons/mail/models/ir_attachment.py
@@ -50,29 +50,22 @@ class IrAttachment(models.Model):
         self.ensure_one()
         return self.env.user.partner_id
 
-    def _attachment_format(self, legacy=False):
+    def _attachment_format(self):
         safari = request and request.httprequest.user_agent and request.httprequest.user_agent.browser == 'safari'
-        res_list = []
-        for attachment in self:
-            res = {
+        return [
+            {
                 'checksum': attachment.checksum,
                 'id': attachment.id,
                 'filename': attachment.name,
                 'name': attachment.name,
                 'mimetype': 'application/octet-stream' if safari and attachment.mimetype and 'video' in attachment.mimetype else attachment.mimetype,
-            }
-            if not legacy:
-                res['originThread'] = [('insert', {
+                'originThread': [('insert', {
                     'id': attachment.res_id,
                     'model': attachment.res_model,
-                })]
-            else:
-                res.update({
-                    'res_id': attachment.res_id,
-                    'res_model': attachment.res_model,
-                })
-            res_list.append(res)
-        return res_list
+                })],
+            }
+            for attachment in self
+        ]
 
     @api.model
     def _get_upload_env(self, request, thread_model, thread_id):
diff --git a/addons/mail/models/mail_message.py b/addons/mail/models/mail_message.py
index 156b7bf35e0..4ab8d098b61 100644
--- a/addons/mail/models/mail_message.py
+++ b/addons/mail/models/mail_message.py
@@ -866,7 +866,7 @@ class Message(models.Model):
     # MESSAGE READ / FETCH / FAILURE API
     # ------------------------------------------------------
 
-    def _message_format(self, fnames, format_reply=True, legacy=False):
+    def _message_format(self, fnames, format_reply=True):
         """Reads values from messages and formats them for the web client."""
         vals_list = self._read_format(fnames)
         thread_ids_by_model_name = defaultdict(set)
@@ -908,7 +908,7 @@ class Message(models.Model):
                 'default_subject': default_subject,
                 'guestAuthor': guestAuthor,
                 'notifications': message_sudo.notification_ids._filtered_for_web_client()._notification_format(),
-                'attachment_ids': message_sudo.attachment_ids._attachment_format() if not legacy else message_sudo.attachment_ids._attachment_format(legacy=True),
+                'attachment_ids': message_sudo.attachment_ids._attachment_format(),
                 'trackingValues': allowed_tracking_ids._tracking_value_format(),
                 'linkPreviews': message_sudo.link_preview_ids._link_preview_format(),
                 'messageReactionGroups': reaction_groups,