Add FileUpload walkthrough

[davidfowl]

From a C# client

https://stackoverflow.com/questions/34547398/when-uploading-files-to-asp-net-5-web-api-the-collection-of-iformfile-is-empty

From a web browser

https://stackoverflow.com/questions/34466141/how-to-upload-file-with-jquery-to-mvc-vnext-webserver