Javascript Validation While Image Uploading

<script>

function Checkfiles()
{
    var fup = document.getElementById('profile_img');
    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 == "doc")
    {
        return true;
    }
    else
    {
        document.getElementById("err_profile_img").innerHTML="Upload a valid image";
        return false;
    }
}

</script>

<input type="file" name="profile_img" id="profile_img" onchange="Checkfiles()"/>