This is usually a server configuration issue, not a problem with the website files themselves.
What to do: Contact your hosting provider and send them this error message:
The most common fixes are:
1. Make sure .htaccess exists in the same folder as index.php (it may be a hidden file).
2. Ask your host to enable mod_mime and mod_rewrite for Apache.
3. Make sure JavaScript files are served with MIME type text/javascript.