How to open downloaded file on iOS

We're developing mobile application in Xamarin. How can next be accomplished in iOS:

• user downloads file from url (http request is made to REST API, secured with Authentication Basic username:secretKey)
• file is saved to iOS
• user opens a file (allowed are jpg, png, pdf, doc, docx, png)
• file is opened in default application (e.g. for images image viewer)

As file operations are platform specific, here's interface definition:

public interface IFileHelper
{
  void DownloadFileAndSave(Models.DocumentModel document);
}

Android implementation:

public class FileHelper : IFileHelper
{
  // download file and view status in download manager
  public void DownloadFileAndSave(Models.DocumentModel document)
  {
    DownloadManager dm = (DownloadManager)Android.App.Application.Context.GetSystemService(Context.DownloadService);
    string url = WebApiUtils.GetBaseUrl() + string.Format("Api/v1/Dms/{0}", document.UniqueId);
    DownloadManager.Request request = new Android.App.DownloadManager.Request(Android.Net.Uri.Parse(url)));

    request.AddRequestHeader("Authorization", "Basic " + WebApiUtils.GetEncodedCredentials(Auth.Users.Current));

    var downloadFile = Android.OS.Environment.GetExternalStoragePublicDirectory(Android.OS.Environment.DirectoryDownloads);
    string path = Path.Combine(downloadFile.AbsolutePath, document.FileName);
    request.SetDestinationUri(Android.Net.Uri.FromFile(new Java.IO.File(path)));
    request.SetMimeType(document.ContentType);  
    request.SetNotificationVisibility(DownloadVisibility.VisibleNotifyCompleted);

    dm.Enqueue(request);
}

In Android file is simply stored on the filesystem and with File Explorer which is by default installed on any Android (i.e. My Files -> device storage -> Download), the file is opened in default application for file's mime type. Everything fine on Android.

Apple iOS implementation:

public class FileHelper : IFileHelper
{
  public void DownloadFileAndSave(Models.DocumentModel document)
  {
    WebClient webClient = new WebClient();

    webClient.Headers.Add(HttpRequestHeader.Authorization, "Basic " + WebApiUtils.GetEncodedCredentials(Auth.Users.Current));
    webClient.DownloadDataAsync(new System.Uri(WebApiUtils.GetBaseUrl() + string.Format(Consts.ApiUrls.GetDocument, document.UniqueId)));

    webClient.DownloadDataCompleted += (sender, e) =>
    {
      byte[] content = e.Result;
      string path = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments), document.FileName);

      // doesn't throw exception therefore saved ok
      File.WriteAllBytes(path, content);

      Uri uri = new Uri(String.Format("file://{0}", path));

      // doesn't work.
      Device.OpenUri(uri);
    };
  }
}

Is there any other way to open downloaded file in default application. If I open url e.g. http://example.com/files/file1.png it opens the file in safari, but I can't put Authorization: Basic headers in Device.OpenUri.

I read about Load Non-Web Documents with WebView but you would have to build each file as BundleResource.