The Upload sends the file in a request identical to a regular form submit (multipart/form-data). The code you'd normally use to handle a form POST will also work with the Upload.
For ASP.NET WebForms that'd be the Request.Files collection.
The TempData is a feature of the ASP.NET MVC framework. It uses the Session to persist data from one request to another, typically after a redirect.
It's used to populate the file list in the example.
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