The 'X Content Type Options' response header tells web browsers to disable MIME and content sniffing. This prevents attacks such as 'MIME confusion attacks'. It will reduce your site's exposure to 'drive-by download' attacks and prevents your server from uploading malicious content that is disguised with clever naming.
To add this security header to your site simply add the below code to your htaccess file:
Header set X-Content-Type-Options "nosniff"