why Upload checks Mime types for allowed file types?

[eluser]eroy4u[/eluser]
Why can't Upload just check the extension for allowed file types?

e.g. supposed I set 'ppt' as the allowed file type,
why don't the Upload library check if 'ppt' is the extension of the file,
instead checking 'application/powerpoint'?

as I think some powerpoint maybe of mime type 'application/octet-stream',
so this will make clients not able to upload a '.ppt' file with mime type 'application/octet-stream.

am I right?