struts2上传文件，如何获取文件的后缀名

  <input name="cmpPic" class="{validate:{required:true,accept:'bmp?|jpg|gif|png'}}" type="file"/>

                 String path=ServletActionContext.getServletContext().getRealPath("/company_image");        File cmpPicSrc= new File(company.getCmpPic());        String newFileName=company.getCmpId()+"_"+System.currentTimeMillis()+".jpg";        File cmpPicDest= new File(path,newFileName);        FileUtils.copyFile(cmpPicSrc, cmpPicDest);