Returning user uploaded file using custom controller

Hi everybody.

I made a custom controller to handle file uploads and I'd like to add the capability to return the file via API made using this repository.
In the function __construct function I added:

$router->register('GET', '/records/file/*', array($this, 'getFileV2'));

In

public function getFileV2(ServerRequestInterface $request): ResponseInterface

I need to get the file, decrypt (because in POST controller function the file is correctly uploaded as encrypted) but also if I add

header("Content-Type: image/png");
return $decryptedFile;

I can't view the file and the error message is:

UploadUserFile::getFileV2(): Return value must be of type Psr\\Http\\Message\\ResponseInterface, string returned

How can I do this?