File upload validation using javascript

<script language="javascript">

function validateFileupload()
{
//Validating FIle SIZE

if((Math.round(document.getElementById("file1").files[0].size)/(1024*1024)) > 4)
            {
                    alert('Document is not UPLOADED. Maximum file size is restricted to 2MB.Please try again Uploading other document!!');
                      return false;
            }


//VALIDATING FILE TYPE

var ext = document.Form1.file1.value;
            
            
              ext = ext.substring(ext.length-3,ext.length);
              ext = ext.toLowerCase();


if(ext.length > 0)
                 {

                    if((ext != 'pdf') && (ext != 'xls') && (ext != 'xlsx') && (ext != 'doc') && (ext != 'docx') && (ext != 'png') && (ext != 'jpg')) {
                    alert('You selected a .'+ext+' file in policy Docs; Only files with following extensions are allowed (pdf,xls,xlsx,doc,docx,png,jpg)!');
                    return false;
                      }

                 
                  else
                    return true;
                  
                }//if

}//FUNC TION END

</script>