If you can't upload documents of type Office 2007 to your Windows server you might want to check if IIS allows Office 2007 mimetypes! In my case I was running Windows Server 2003 and IIS 5.6.

Use this guide to correct the problem: http://www.bram.us/2007/05/25/office-2007-mime-types-for-iis/

Keywords: Mimetype, docx, xlsx, Office 2007, IIS 5.6, uploading office documents, Windows 2003 Server.