Update MIME types for Microsoft Office files.

This is a tiny pull request to fix one incorrect MIME type and add two others. These are very common files types.

Fixed the MIME type for ".docx" files.
Added MIME types for "xlsx" and "pptx" files.

For a complete list checkout this [TechNet article](http://technet.microsoft.com/en-us/library/ee309278%28office.12%29.aspx).