JavaScript实现文件上传控件的图片类型校验

<script language="javascript">
function Checkfiles()
{
var fup = document.getElementById('logo1');
var fileName = fup.value;
var ext = fileName.substring(fileName.lastIndexOf('.') + 1);
if(ext == "gif" || ext == "GIF" || ext == "JPEG" || ext == "jpeg" || ext == "jpg" || ext == "JPG" || ext == "png" || ext == "PNG")
{
return true;
}
else
{
alert("Upload JPG, JPEG, PNG, GIF images only");
fup.focus();
return false;
}
}
</script>
onchange="Checkfiles()"