They render the uploaded files themselves using the content of files, rendered in native HTML. Browsers are unable to render special file types, such as PDF, MS Word and MS Excel etc. You need either a plugin to be installed on the browser, or you need to render it using HTML, and for that you need to read everything from file and then render it using tables, gridviews etc. Otherwise browser will simply download the file to disk.
Your best bid is to use an Excel to HTML converter, that converts your Excel sheet data to an HTML table element. You can find hundreds of them online.
Google[
^] will help. There are special libraries created too,
.NET Office[
^] etc that can be utilized here. But choice is yours.