This week's book giveaway is in the Programmer Certification forum. We're giving away four copies of OCP Oracle Certified Professional Java SE 11 Programmer I Study Guide: Exam 1Z0-815 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
If wb.write(fileOut) writes the XLS file you still need to *open it* if you want to return the data to the browser. Or perhaps you could wb.write(...) directly to the response--depends on what you're doing.
But you never open a file for reading here, nor do you send anything to the browser.