Web sites that use an Excel 97-2003 MIME type to open conent streamed from the web server may find that users recieve the following alert message when the file attempts to open in Microsoft Excel 2007:

"The file you are trying to open, '[filename]', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?"  (Yes | No | Help)

This problem and the related information is documented in the following post:
Excel 2007 Extension Warning On Opening Excel Workbook from a Web Site