文件上传时 js判断文件大小是否合法

var isIE =
/msie/i.test(navigator.userAgent) && !window.opera;       

   function fileChange(target) {   

      document.getElementById("forms").disabled=false;

     var fileSize = 0;        

     if (isIE && !target.files) {    

       var filePath = target.value;    

      var fileSystem =
new ActiveXObject("Scripting.FileSystemObject");       

       var file = fileSystem.GetFile (filePath);    

       fileSize = file.Size;

     } else {   

      fileSize = target.files[0].size;

console.log(fileSize);   

      }  

      var size = fileSize / 1024;

      if(size>10000/5){

       alert("附件不能大于2M");

       //禁用按钮

       $("#forms").attr({"disabled":"disabled"});

       return;

      }

}