js前端验证上传的文件大小及格式

js前端验证上传的文件大小及格式
<script type="text/javascript">
        function fileChange(target) {
            var fileSize = 0;
            if (!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;
            }

            var size = fileSize / 1024;
            if (size <110) { //空白的Excel表最大为10.1KB，当小于它及判断为Excel内容为空
                alert("导入的Excel表不能为空！");
                target.value = "";
                return
            } 
            if (size > 5000) {
                alert("导入的Excel表不能大于5M！");
                target.value = "";
                return
            }            
            var name = target.value;
            var fileName = name.substring(name.lastIndexOf(".") + 1).toLowerCase();
            if (fileName != "xlsx" && fileName != "xls") {
                alert("请选择.xlsx格式文件上传！");
                target.value = "";
                return
            }
        } 
    </script>

 <td><input type="file" id="file" name="file" accept="application/vnd.ms-excel,application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" onchange="fileChange(this);"></td>