If you're talking about uploading an xltx file from within mojoPortal, you can add that extension to this key in user.config to allow that:
<add key="AllowedUploadFileExtensions" value=".cer|.gif|.jpg|.jpeg|.png|...|.xltx">
If you want this file to be stored on your site and served by IIS, you'll need to open IIS Manager and add that extension to the MIME types for the site. If you're running in a hosted environment, you may need to contact your hosting provider to do this portion.
Jamie