Added MIME types for .ppt uploads

I was not able to upload a .ppt (1997 - 2003 compatible) exported from Keynote.  Checking the MIME type under OSX using <code>file --mime-type</code> on the file I get back "application/vnd.ms-office".  My web server identifies the file as application/msword.  I have added these both to the ppt array in mimes.php.