title: addFlashAttribute and holding data
body: <p>How should I hold data from POST request when I use addFlashAttribute?</p>
  
  <pre><code>@GetMapping("/collage")
  public String paintPicture(@ModelAttribute(value="picture") String img){
  
      //How to  hold 'img' here?
      //When I send GET I want to see the image again (not only after the  redirect).
  
      return "collage";
  }
  
  @PostMapping(value="/sending")
  public String redirect(@RequestParam(value="image") MultipartFile img,  RedirectAttributes redirectAttr) throws IOException {
  
      String imgAsBase64 = Base64.encodeBase64String(img.getBytes());
      redirectAttr.addFlashAttribute("picture",imgAsBase64);
          return "redirect:/collage"; 
  }
  </code></pre>