Notable files:

Forked from https://raw.githubusercontent.com/dotnet/macios/f1ae8365b67edea7c80e17e6d5efb66c4253b4d8/src/Foundation/NSUrlSessionHandler.cs but otherwise essentially unchanged apart from making it compile and changing namespace

Additional members added to NSUrlSessionHandler to support FileDownload

Added the following to resume and react to completed file download while app is in the background:

[Export("application:handleEventsForBackgroundURLSession:completionHandler:")]
public virtual void HandleEventsForBackgroundUrlCompletion(UIApplication application, string sessionIdentifier, Action completionHandler)
{
    // ...
}

The new forked NSUrlSessionHandler has a new DownloadFileAsync method which will download the file and put it on disk to the provided location. It'll return an HttpResponse just like SendAsync, but the HttpResponse.Content will be of type NSUrlDownloadFileContent which contains the path to the downloaded file. See src\MainPage.xaml.cs for example