IT ...

When people try to upload PowerPoint files, it says extension mismatch or something.
In LocalSettings.php, .ppt is already allowed extension.

It turned out to be "file" command in Linux/Unix tends to identify .ppt file as "Microsoft Installer".

A quick solution, disable extension verification.
$wgStrictFileExtensions = false;

We have disabled MimeType checking also since it's for internal user only.
$wgVerifyMimeType = false;