title: FileOpenPicker with appium unittest
body: <p>How can I handle a FileDialog in appium for a C# UWP app?</p>
  
  <p>In my case, the user clicks on a button and the FileOpenPicker prompts, and the user has to select one or more files. How can I handle this FileOpenPicker in a appium UnitTest?</p>
  
  <pre><code>var ButtonPlusElement = AppSession.FindElementByAccessibilityId("ButtonFilePicker");
  Assert.IsNotNull(ButtonPlusElement);
  _ButtonFilePicker = ButtonPlusElement;
  _ButtonFilePicker.Click(); // FileOpenPicker appears
  //how set the selected file of the FilePicker?
  </code></pre>
  
  <p>Any help appreciated. Thanks.</p>