Fix: change the return type.

Author: ya861220

system/HTTP/DownloadResponse.php

@@ -132,10 +132,8 @@ public function getContentLength(): int
 
     /**
      * Set content type by guessing mime type from file extension
-     *
-     * @return void
      */
-    private function setContentTypeByMimeType()
+    private function setContentTypeByMimeType(): void
     {
         $mime    = null;
         $charset = '';