If you are trying to copy a file into a SharePoint 2010 site using SharePoint Designer 2010, there’s an easy fix for it.

All you need to do is to go to Central Admin for that site. Once you’re there, click on ‘Security’. Then, under the ‘General Security’ section, there’s a link that says ‘Define blocked file types’. Once you click on that link and get to the page, look in the list to find your file extension. All you need to do is to remove it from the list and click the ‘OK’ button down below that list.