Android : Android: Get file from PHP Server by HttpResponse

My android application successfully sends files to a remote php server. Until know the server responds by sending back a simple string:

PHP Code

// if a file was posted
if($_FILES && $_POST){

    if(move_uploaded_file($file, $dir)) {
         echo "File successfully sent to server.";
    }

I can get this text answer with this peace of code:

Java Code

HttpResponse response = httpClient.execute(httpPost);
HttpEntity resEntity = response.getEntity();

// get server response
if (resEntity != null)  {    
    return EntityUtils.toString(resEntity).trim();
}

Now I don't want to get a simple text response, but i expect a whole file. I don't have much experience and got stuck:

PHP Code

if(move_uploaded_file($app, $upload_file . $file_ending)) {
    readfile($res_file); // read file and write it output buffer
}

Java Code

HttpResponse response = httpClient.execute(httpPost);
HttpEntity resEntity = response.getEntity();

if (resEntity != null)  
    return String.valueOf(resEntity.getContentLength()); 

getContentLength returns 14, which is definitely not the file size in byte.

How do I get a file as response from a PHP server?