The sender may have used an ISP (internet service provider) which doesn't see Final Draft's unique file extention, .fdx or .fdr, as a valid file format. AOL in particular is known to have this issue. The ISP has changed the file type and Final Draft won't be able open it unless you convert it into a readable format. To avoid this situation in the future, tell the person who's sending you a script to compress it before attaching it to an email. For more information on file compression, click HERE.