After applying cumulative site update 14 to 9.00 SP2 or upgrading to 9.10, form reports do not automatically open in Microsoft Excel.
Opening form reports in Excel 2013 would generate the error The file format and extension of 'form_data_export-1.xls' don't match. The file could be corrupted or unsafe.
- 73824 - If you exported a form report and attempted to open it in Excel 2010 or 2013, an error was returned by Excel indicating the file could be corrupted or unsafe.
You can bypass this error and open the file, but in some cases Excel would crash due to the error. To alleviate this, a cumulative update was created, which caused the report to be generated as an HTML file.
To open the file in Excel either start in Excel and open it there or change HTML files default program.
To change the default program, right click on the file in File Explorer, click Properties , click Change , and change it to Excel.