Create File Upload validator in Java Script

var FUpload = document.getElementById('<%= fileUpl.ClientID %>');
var myfile = FUpload.value;
if (myfile == "")
{
alert('Please select any file!');
return false;
}
else
{
if (myfile.indexOf("pdf") < 0)
{
alert('Please select PDF file!');
return false;
}

}