Hi Flip,
Thank you for contacting Syncfusion support.
XlsIO doesn’t have support to open the file created before it is saved. However, we have another control Spreadsheet in web platform. You can achieve your requirement by saving the created file as stream and load the saved file in spreadsheet control. Kindly refer the below link to know more about this.
You can also refer to online documentation regarding spreadsheet control from the following link.
Regards,
Abirami.