diff --git a/addons/mail/static/tests/helpers/mock_server.js b/addons/mail/static/tests/helpers/mock_server.js
index 4ad066ba732..9f49d6c24df 100644
--- a/addons/mail/static/tests/helpers/mock_server.js
+++ b/addons/mail/static/tests/helpers/mock_server.js
@@ -67,7 +67,7 @@ MockServer.include({
     async _performFetch(resource, init) {
         if (resource === '/mail/attachment/upload') {
             const ufile = init.body.get('ufile');
-            const is_pending = init.body.get('is_pending');
+            const is_pending = init.body.get('is_pending') === 'true';
             const model = is_pending ? 'mail.compose.message' : init.body.get('thread_model');
             const id = is_pending ? 0 : parseInt(init.body.get('thread_id'));
             const attachmentId = this.mockCreate('ir.attachment', {