js判断用户上传文件类型

<script type="text/javascript">
    //指定允许的上传文件类型
    var extArray = new Array(".doc",".docx",".wps",".dot");
    function LimitAttach(form, file){
        var allowSubmit = false;
        if (!file){
            return;
        }
        while (file.indexOf("") != -1){
            file = file.slice(file.indexOf("") + 1);
        }
        var ext = file.slice(file.indexOf(".")).toLowerCase();
        for (var i = 0; i < extArray.length; i++) {
            if (extArray[i] == ext){
                allowSubmit = true;
                break;
            }
        }
        if (allowSubmit){
            form.submit();
        }else{ //检测上传文件类型
            alert("只能上传以下格式的文件:"+ (extArray.join("")) + "n请重新选择再上传.");
        }
    }
</script>

<form method="post" name="upform" action="">
    <input type="file" name="uploadfile" >
    <input type="button" name="Submit" onclick="LimitAttach(this.form,this.form.uploadfile.value)" value="Submit">
</form>