SilverstripeDownloadController.php

<?php
...
$download = $request->getVar("download") ? true : false;
  if ($download) {
      // Get the the product file path,name,type, and size
      $productFile = $product->FullUpload()->getAbsoluteURL();
      $productFileName = $product->FullUpload()->getFilename();
      $productFileType = $product->FullUpload()->getMimeType();
      $productFileSize = $product->FullUpload()->getAbsoluteSize();
      // Update the HTTPResponse object with the proper download headers
      $this->returnDownloadResponse();
      $this->setResponse(new HTTPResponse());
      $response = $this->getResponse();
      $response->setStatusCode(200);
      $response->setBody(readfile($productFile));
      $response->addHeader("Content-Disposition", 'attachment;filename="'.$productFileName.'"');
      $response->addHeader("Content-Type", $productFileType);
      $response->addHeader("Content-Length", $productFileSize);
      // Return the HTTPResponse object
      return $response;
  }
...