Martin Weissen wrote :
$wgEnableUploads = true; $wgFileExtensions = array( 'png', 'gif', 'jpg', 'jpeg', 'ppt', 'pdf',...
all I can say is "it works for me" with the same parameters in LocalSettings.php
you can try to put $wgMimeDetectorCommand= "file -bi"; in LocalSettings.php in case there is a problem with the mime detector of Mediawiki
(you must have "file" installed of course)