【Unsolicited PR】I changed the download method to testable.

system/CodeIgniter.php

@@ -35,6 +35,7 @@
  * @since	Version 3.0.0
  * @filesource
  */
+use CodeIgniter\HTTP\DownloadResponse;
 use CodeIgniter\HTTP\RedirectResponse;
 use CodeIgniter\HTTP\Request;
 use CodeIgniter\HTTP\ResponseInterface;
@@ -93,7 +94,7 @@ class CodeIgniter
 
 	/**
 	 * Current response.
-	 * @var HTTP\Response
+	 * @var HTTP\ResponseInterface
 	 */
 	protected $response;
 
@@ -326,6 +327,7 @@ protected function handleRequest(RouteCollectionInterface $routes = null, $cache
 		// so it can be used with the output.
 		$this->gatherOutput($cacheConfig, $returned);
 
+		$filters->setResponse($this->response);
 		// Run "after" filters
 		$response = $filters->run($uri, 'after');
 
@@ -890,6 +892,10 @@ protected function gatherOutput($cacheConfig = null, $returned = null)
 			ob_end_clean();
 		}
 
+		if ($returned instanceof DownloadResponse) {
+			$this->response = $returned;
+			return;
+		}
 		// If the controller returned a response object,
 		// we need to grab the body from it so it can
 		// be added to anything else that might have been

system/Exceptions/DownloadException.php

@@ -0,0 +1,35 @@
+<?php namespace CodeIgniter\Exceptions;
+
+/**
+ * Class DownloadException
+ *
+ * @package CodeIgniter\Exceptions
+ */
+class DownloadException extends \RuntimeException implements ExceptionInterface
+{
+
+	public static function forCannotSetFilePath(string $path)
+	{
+		return new static(lang('HTTP.cannotSetFilePath', [$path]));
+	}
+
+	public static function forCannotSetBinary()
+	{
+		return new static(lang('HTTP.cannotSetBinary'));
+	}
+
+	public static function forNotFoundDownloadSource()
+	{
+		return new static(lang('HTTP.notFoundDownloadSource'));
+	}
+
+	public static function forCannotSetCache()
+	{
+		return new static(lang('HTTP.cannotSetCache'));
+	}
+
+	public static function forCannotSetStatusCode(int $code, string $reason)
+	{
+		return new static(lang('HTTP.cannotSetStatusCode', [$code, $reason]));
+	}
+}

system/Filters/Filters.php

@@ -91,6 +91,11 @@ public function __construct($config, RequestInterface $request, ResponseInterfac
 	{
 		$this->config = $config;
 		$this->request = & $request;
+		$this->setResponse($response);
+	}
+
+	public function setResponse(ResponseInterface $response)
+	{
 		$this->response = & $response;
 	}