diff --git a/addons/website/controllers/form.py b/addons/website/controllers/form.py
index 706bdae3471..5c87a5b68b0 100644
--- a/addons/website/controllers/form.py
+++ b/addons/website/controllers/form.py
@@ -261,7 +261,11 @@ class WebsiteForm(http.Controller):
             }
             attachment_id = request.env['ir.attachment'].sudo().create(attachment_value)
             if attachment_id and not custom_field:
-                record.sudo()[file.field_name] = [(4, attachment_id.id)]
+                record_sudo = record.sudo()
+                value = [(4, attachment_id.id)]
+                if record_sudo._fields[file.field_name].type == 'many2one':
+                    value = attachment_id.id
+                record_sudo[file.field_name] = value
             else:
                 orphan_attachment_ids.append(attachment_id.id)