Adding this code to LocalSettings.php will allow you to upload different filetypes to your wiki.
$wgFileExtensions = array_merge( $wgFileExtensions, array( 'pdf', 'ppt', 'doc', 'docx', 'xls', 'xlsx' ) );
To allow uploads you also need to change the owner of the folder to www-data.
# type in shell chown -R www-data /var/www/wiki/images/
References:
mediawiki.org/wiki/Manual:$wgFileExtensions
mediawiki.org/wiki/…/Trouble_uploading_after_installation
