Have spent HOURS trying to figure out why I get the above error, "Excel cannot open this file. The file format or file extension is not valid." every time I try to view an XLSX/PPTX/DOCX file in an email attachment.
I've completely uninstalled Outlook 2011, done system traces to see what I could find, view all sorts of logs, re-installed multiple times, etc. Even upgraded to Lion.
Turns out, it was the NAME of my hard drive causing the issue. Go figure.
To check this, go into Disk Utility and see what the name of your HD is. If it doesn't have ABC123 characters, change the name. In the example below, it just shows ":" as the name:
To change the name of the drive, just RIGHT CLICK and select SHOW IN FINDER, then rename it as you would any normal file.
