Download Bug with Powerpoint .pptx - open file program not recognised

Hi There,
Wondering if anyone has encountered this problem before.
I upload a microsoft .pptx file that can then be downloaded via the website. When someone then downloads the file it isn't recognised as a powerpoint file so you have to manually select which program to open the file. This only happens for the powerpoint files the .docs are fine?

Any help would be Ace.
Thanks