javascript 檢測上傳圖片類型代碼

<form enctype="multipart/form-data" method="post" name="upform" target="upload_target" action="upload.php">
  <input type="file" name="Filedata" id="Filedata"/>
  <input style="margin-right:20px;" type="submit" name="" value="上傳形象照" onclick="return checkFile();" /><span style="visibility:hidden;" id="loading_gif"><img src="loading.gif" align="absmiddle" />上傳中，請稍侯......</span>
  </form>

上面國html頁面

 function checkFile()
    {
     var path = document.getElementById('Filedata').value;
     var ext = getExt(path);
     var re = new RegExp("(^|\s|,)" + ext + "($|\s|,)", "ig");
      if(extensions != '' && (re.exec(extensions) == null || ext == '')) {
     alert('對不起，只能上傳jpg, gif, png類型的圖片');
     return false;
     }
     showLoading();
     return true;
    }