code: @Override
      protected void buildJsonItemSpecificPart(JSONObject jsonObj, CmsResource res, String sitePath) {
  
          // file target
          String pointer;
          try {
              pointer = new String(getCms().readFile(res).getContents());
              if (CmsStringUtil.isEmptyOrWhitespaceOnly(pointer)) {
                  pointer = getJsp().link(getCms().getSitePath(res));
              }
              jsonObj.append("pointer", pointer);
          } catch (CmsException e) {
              // reading the resource or property value failed
              LOG.error(e.getLocalizedMessage(), e);
          } catch (JSONException e) {
              if (LOG.isErrorEnabled()) {
                  LOG.error(e.getLocalizedMessage(), e);
              }
          }
  
      }
docs: Fills the JSON object with the specific information used for pointer file resource type.<p>
  
  <ul>
  <li><code>pointer</code>: the content of the pointer resource. This could be an external or internal link.</li>
  </ul>
  
  @see org.opencms.workplace.galleries.A_CmsAjaxGallery#buildJsonItemSpecificPart(JSONObject jsonObj, CmsResource res, String sitePath)