Fix placement of default response in system_file_download function.

core/modules/system/system.module

@@ -587,8 +587,6 @@ function system_file_download($uri): Response {
         if (array_intersect($parts, ['.', '..'])) {
           return -1;
         }
-        // Default response when the file is not found.
-        return new Response('File not found', 404);
       }
     }
   }
@@ -605,6 +603,9 @@ function system_file_download($uri): Response {
       ];
     }
   }
+
+  // Default response when the file is not found.
+  return new Response('File not found', 404);
 }
 
 /**