When opening a file (with .xls file extension) in my web browser (IE6), it delivers an error "because the file format or file extension is not valid" msg. When the file is saved, it can be opened in Excel '03, but not Excel '07. I need for clients to be able to view in Excel '07. What is causing this error, and how can it be fixed? NOTE: Changing the filename to .xlsx does not help. File attached.