Error opening the file
Here is a problem with Excel and mimetypes, this result in a dialog box with the following content in Excel.“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)
You have to select yes to this. More information about the problem can be found at http://blogs.msdn.com/vsofficedeveloper/pages/Excel-2007-Extension-Warning.aspx. It is not possible to solve the problem.