why Upload checks Mime types for allowed file types?

[eluser]gtech[/eluser]
Just because a file is named .ppt does not mean it is a powerpoint presentation. for example if you named a Jpeg image.jpg to image.ppt the file will still be a jpeg. the extension just tells the operation system what application to open when viewing.. if you loaded the image.ppt in a art package the jpeg will still appear.