Added Microsoft MIME Types + Force Download Options

Adding microsoft MIME types and Force download for some files avoid bugs (for example, xlsx opened in Firefox, etc.).

See references : http://www.amember.com/forum/threads/force-file-downloads-with-htaccess.14980/
http://www.saelboat.com/blog/2012/12/htaccess-guide-force-downloads-of-mp3s-pdfs-and-other-media-files-299