Hello,
We are creating a workbook in memory passing the file format type Xlsx in the constructor, than write to a worksheet then upload the workbook to our database as binary without saving it to the filesystem. During our process we download the binary from the database and upload it to SharePoint 2010 with a .xlsx extension without saving it to the filesystem. When we open the workbook from the SharePoint site excel throws an error saying that the format or the extension is incorrect. When we upload the workbook to SharePoint with an .xls extension it works fine. Is there anyway to have the workbook created with the xlsx format?