diff --git a/src/ifctester/webapp/serve.py b/src/ifctester/webapp/serve.py
index 87e92930f3..3eaf48d35c 100644
--- a/src/ifctester/webapp/serve.py
+++ b/src/ifctester/webapp/serve.py
@@ -18,8 +18,13 @@
 
 import os
 import argparse
+import mimetypes
 from flask import Flask, send_from_directory, send_file
 
+# Browsers expecting this specific MIME type for .mjs files,
+# otherwise they fail to load.
+mimetypes.add_type("text/javascript", ".mjs")
+
 app = Flask(__name__)