Excel cannot open the file 'filename.xlsx' because the file format for the file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file

When you try to open a workbook that you receive in an e-mail message for review in Microsoft Office Excel 2007, the workbook does not open. Additionally, you receive the following error message:
Excel cannot open the file 'filename.xlsx' because the file format for the file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.
This issue occurs only when you send a workbook for review. The workbook file is saved as an Excel binary workbook. However, the workbook has an .xlsx extension that is appended to the file.

To work around this issue, save the workbook as one of the following file formats before you send the workbook for review:
--Excel Binary Workbook (*.xlsb)
--Excel 97-2003 Workbook (*.xls)