added 'application/zip' identifier to pptx mime-type's array in mimes.php

This 'application/zip' type that the .pptx file was being identified as needed to be added to the .pptx type's array in mimes.php...  This fixed the issue that one school was having because they are on a different version of php/linux and identifying a pptx mime-type on their system returns it as 'application/zip'.  Some versions of Linux/PHP return the mime type correctly...