Office 2007 has introduced new file types for Word, PowerPoint and Excel i.e. docx, pptx, and xlsx.
See discussion in http://moodle.org/mod/forum/discuss.php?d=71960#p321981

get_mime_types_array() in /lib/filelib.php should be updated to include the following (or similar) lines:

'docx' => array ('type'=>'application/msword', 'icon'=>'word.gif'),

'pptx' => array ('type'=>'application/vnd.ms-powerpoint', 'icon'=>'powerpoint.gif'),

'xlsx' => array ('type'=>'application/vnd.ms-excel', 'icon'=>'excel.gif'),