File download in C++ metro style application

Question

I am trying to write an application which can download a file from a server I am using a file save picker along with the c++ BackgroundDownloader

since there may be different files from the server I want to append the file extension dynamically by splitting the specified URI .Below is the code I am using for this.

  Uri^ source = ref new Uri(StringTrimmer::Trim(serverAddressField->Text));
	  String^ filenameExt= serverAddressField->Text;

	  const wchar_t* pntr=filenameExt->Data();
	
	  int l= filenameExt->Length();

	  auto temp =std::map<wchar_t,String^>();
	  auto test= temp.find(filenameExt->Data()[l-3]);


	   String^ destination1 = StringTrimmer::Trim(fileNameField->Text);
	  FileSavePicker^ savePicker = ref new FileSavePicker();
        savePicker->SuggestedStartLocation = PickerLocationId::DocumentsLibrary;
		
        auto plainTextExtensions = ref new Platform::Collections::Vector<String^>();
        plainTextExtensions->Append(".pdf");
		plainTextExtensions->Append(".txt");
		 savePicker->FileTypeChoices->Insert("Portable Document file", plainTextExtensions);
        savePicker->FileTypeChoices->Insert("Plain text", plainTextExtensions);
        savePicker->SuggestedFileName = "New Document";

        create_task(savePicker->PickSaveFileAsync()).then([this](StorageFile^ file)
        {
		Uri^ source = ref new Uri("http://www.microsoft.com/about/mspreview/windows8/Windows8_RP_Product_guide.pdf");
			 BackgroundDownloader^ downloader = ref new BackgroundDownloader();
			 DownloadOperation^ download = downloader->CreateDownload(source, file);
      /*  BackgroundDownloader^ downloader = ref new BackgroundDownloader();
        DownloadOperation^ download = downloader->CreateDownload(source, destinationFile);*/

			 Output->Text="Downloading " + source->AbsoluteUri + " to " + file->Name + ", " + download->Guid;

        // Attach progress and completion handlers.
        HandleDownloadAsync(download, true);

I tried to split the string using the following code but its not working properly. 

  const wchar_t* pntr=filenameExt->Data();

  int l= filenameExt->Length();

  auto temp =std::map<wchar_t,String^>();
  auto test= temp.find(filenameExt->Data()[l-3]);

Don't mind for the silly question. As this is the first time I am working with c++ .