Dynamically determine file types through links

1, Determine the type of link ： 

// The judgment is the picture 、 video 、ex、word One of them 
    function checkform(f_content){
      console.log(" link ："+f_content);
      var fileext=f_content.substring(f_content.lastIndexOf("."),f_content.lastIndexOf("?"));
      fileType=fileext.toLowerCase();
      console.log(fileType);
      if(
          fileType == ".jpg" || fileType == ".JPG" || fileType == "image/jpeg" ||
          fileType == ".png" || fileType == ".PNG" || fileType == "image/png"  ||
          fileType == ".gif" || fileType == ".GIF" || fileType == "image/gif"
      ){
        return 0;
      }
      if (fileType=='.mp4'||fileType=='.m2v'||fileType=='.mkv'){
        return 1;
      }
      if (fileType=='.xls'||fileType=='.xlsx'){
        return 2;
      }
      if (fileType=='.doc'||fileType=='.docx'){
        return 3;
      }
      return -1;
    }

2, The echo ： If it's pictures and vidoe Just show , Otherwise, directly through a Click the note to download

              // Incoming link judgment 
              var type = checkform(document);
              console.log("-===============:"+type);
              if(type==0){
                dom += '<img style="width: 100%;margin-bottom: 10px" src="'+document+'">'
              }else if(type==1){
                dom += '<video th:src="'+document+'" controls\n'
                    + '                           style="width: 100%;margin-bottom: 10px"></video>'
              }else if(type==2){
                //dom += '<iframe id="excelIframe" src=https://view.officeapps.live.com/op/view.aspx?src="'+document+'" width=\'100%\' height=\'100%\' frameborder=\'1\'></iframe>'
                var image = 'https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1575615615720&di=94fe53f0421941f41cf9d21527e5f093&imgtype=0&src=http%3A%2F%2Fwww.szrcpx.com%2Fuploads%2F181203%2F1-1Q203161202104.jpg';
                dom+='<img style="width: 40px;margin-bottom: 10px" src="'+image+'"><a href="'+document+'" \n'
                    + '\t\t \t\tdownload="test.png"> Click to download </a>\n';

              }else if(type==3){
                var image = 'http://img3.imgtn.bdimg.com/it/u=3838238180,3232741685&fm=26&gp=0.jpg';
                dom+='<img style="width: 40px;margin-bottom: 10px" src="'+image+'"><a href="'+document+'" \n'
                    + '\t\t \t\tdownload="test.png"> Click to download </a>\n';
              }else{
                // Can't find 
              }

According to the effect ：