JS判断上传图片的格式是否正确

function checkinfo()
  {
  var uploadphoto=document.getElementById("uploadPhoto");
  var str = uploadphoto.value;
  var tt = false;

  if(uploadphoto.value=="")
  {
  alert('请添加上传的图片!');   
  return false;
  }
  if(str.length>0)
  {
  var astr = str.split(".");   
var exp = astr[astr.length-1];   
if(exp == "gif" || exp == "GIF" || exp == "jpg" ||exp == "JPG")
{   
  
return true;  
}
else
{
alert("上传格式不正确，上传格式应为gif或者jpg");
return false;
}
  }   
    
  return true;
  }