title: CKEditor 5 upload image, What information does the upload image return?
body: <p>This is web code:</p>
  
  <pre><code>DecoupledEditor
          .create( document.querySelector( '#webDetails' ),{
              language: 'zh-cn',
              image: {
                  toolbar: [ 'imageTextAlternative' ],
                  styles: [ 'full', 'side' ]
              },
              ckfinder: {
                  uploadUrl: '&lt;%=WEBPATH%&gt;/platform/updateMaterial'
              }
          } )
          .then( editor =&gt; {
              const toolbarContainer = document.querySelector( '#toolbar-webDetails' );
              toolbarContainer.appendChild( editor.ui.view.toolbar.element );
  
          } )
  </code></pre>
  
  <p>This is Spring controller: </p>
  
  <pre><code>@PostMapping("updateMaterial")
  @ResponseBody
  public String updateMaterial(@RequestParam("upload") MultipartFile file, HttpServletRequest request){
      String trueFileName = null;
      String realPath  = null;
      try {
          realPath = request.getSession().getServletContext().getRealPath("/upload");
          System.out.println(realPath);
          trueFileName = uploadImg(realPath, file);
      } catch (IllegalStateException | IOException e) {
          e.printStackTrace();
      }
      return "{\"default\":\"" + realPath + File.separator + trueFileName + "\"}";
  }
  </code></pre>
  
  <p>Here I return the address of the image on disk.
  It is json String style. I want CKEditor 5 api to return the information, but still failure.</p>
  
  <p>What do I need to return in the background to succeed, or am I missing the step?
  thank you.</p>
  
  <p><a href="https://i.stack.imgur.com/fs1fP.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fs1fP.png" alt="enter image description here"></a></p>