FileOpenPicker / FileSavePicker

Pick a File

FileOpenPicker openPicker = new FileOpenPicker();
openPicker.ViewMode = PickerViewMode.Thumpnail;
openPicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
openPicker.FileTypeFilter.Add(".jpg");
openPicker.FileTypeFilter.Add("png");
StorageFile file = await openPicker.PickSingleFileAsync();
if(file != null)
{
  // ...
}

Save a File

FileSavePicker savePicker = new FileSavePicker();
savePicker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;
savePicker.FileTypeChoices.Add("Plain Text", new List<string>() {".txt"});
savePicker.SuggestedFileName = "New Doc";
StorageFile file = await openpicker.PickSaveFileAsync();
//  save content ...