var openFileDialog = new OpenFileDialog
{
     InitialFileName = "test",
     Title = "Title",
     Filters = new System.Collections.Generic.List<FileDialogFilter>()
     {
          new FileDialogFilter()
          {
               Extensions = new System.Collections.Generic.List<string>() {".mp3", ".avi"}
          }
     }
};
_pathButton.Content = openFileDialog.ShowAsync().Result[0];