how to validate image extention by javascript?

function validate()
{
    var filename = document.getElementById("pimage").value;
  
        if(filename!='')
        {
        var ext = getExt(filename);
        if(ext == "jpg" || ext == "jpeg" || ext == "gif" || ext == "png" || ext == "")
        return true;
        alert("Please upload image files only.");
        return false;
        }
    }
  
    function getExt(filename)
    {
        var dot_pos = filename.lastIndexOf(".");
        if(dot_pos == -1)
            return "";
        return filename.substr(dot_pos+1).toLowerCase();
    }
}