<SCRIPT LANGUAGE="JavaScript">
<!--
// 去左空格
function ltrim(s){ 
 return s.replace( /^\s*/, ""); 
}

// 去右空格
function rtrim(s){ 
 return s.replace( /\s*$/, ""); 
}

// 去左右空格
function trim(s){ 
 return rtrim(ltrim(s)); 
}

//获取文件扩展名
function get_ext(f_path){
 var ext = '';
 
 if(f_path != null && trim(f_path).length > 0){
    f_path = trim(f_path);
    ext = f_path.substring(f_path.lastIndexOf(".") + 1, f_path.length);
 }

 return ext;
}

//验证文件扩展名
function chk_ext(f_path){
 var ext = get_ext(f_path);
 
 //根据需求定制
 var accept_ext = new Array("BMP","JPEG","GIF","PSD","PNG","TIFF","TGA","EPS","JPG");
 
 var flag = false;
 
 if(ext != ''){
    for(var i=0; i<accept_ext.length; i++){
  if(ext.toLowerCase() == accept_ext[i])
   flag = true;
    }
 }
 return flag;
}
//-->
</SCRIPT>

调用 

<script type="text/javascript">
 function dd(){
  var name=document.getElementById("yzname");
  if(name.value!=""){
   if(!chk_ext(document.all.file1.value)){
    alert("不支持此格式！");
    return false;
   }
   document.form1.action="../../../upload?path=edu&actionClass=net.hlj.eduWork.myDocuments.seal.service.SealUploadAction";
   document.form1.method="post";
         document.form1.submit();
       // alert("新增成功！");
        }else{
         alert("电子印章名称不能为空！添加失败！");
        }
 }
</script>