如何从ASP.Net Server打开.xlsx文件？

I'm trying to open up .xlsx files generated from our servers because I've been requested to change the files from .xls to .xlsx. There was no problem with the old version, they just wanted to upgrade. I've tried and I've searched many tips and sites where people are asking the same question but no one gave them a correct answer and the latest I've found was around 2012. Here is most likely the main problem.I'm trying to open up .xlsx files generated fro