[FIX] base,mail: do not include field attachments in attachment_count

The _search was already excluding these attachments but not the read_group
Avoid the read_group from mail to search on all existing attachments

Fixes odoo/odoo#27673

closes odoo/odoo#27698

addons/mail/models/mail_thread.py

@@ -239,7 +239,7 @@ class MailThread(models.AbstractModel):
 
     @api.multi
     def _compute_message_attachment_count(self):
-        read_group_var = self.env['ir.attachment'].read_group([('res_model', '=', self._name)],
+        read_group_var = self.env['ir.attachment'].read_group([('res_id', 'in', self.ids), ('res_model', '=', self._name)],
                                                               fields=['res_id'],
                                                               groupby=['res_id'])
 

odoo/addons/base/models/ir_attachment.py

@@ -381,6 +381,15 @@ class IrAttachment(models.Model):
             if not (self.env.user._is_admin() or self.env.user.has_group('base.group_user')):
                 raise AccessError(_("Sorry, you are not allowed to access this document."))
 
+    @api.model
+    def read_group(self, domain, fields, groupby, offset=0, limit=None, orderby=False, lazy=True):
+        """Override read_group to add res_field=False in domain if not present."""
+        if not any(item[0] in ('id', 'res_field') for item in domain):
+            domain.insert(0, ('res_field', '=', False))
+        return super(IrAttachment, self).read_group(domain, fields, groupby,
+                                                    offset=offset, limit=limit,
+                                                    orderby=orderby, lazy=lazy)
+
     @api.model
     def _search(self, args, offset=0, limit=None, order=None, count=False, access_rights_uid=None):
         # add res_field=False in domain if not present; the arg[0] trick below