RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\\.)freespacemods.net/.*$ [NC]
RewriteRule \.(7z|gif|jpg|jpeg|bmp|zip|rar|mp3|flv|swf|xml|php|png|css|pdf)$ - [F]