How to restrict the empty file upload control in asp.net

Using JQuery I am restricting the file extension but I am getting an issue is that "If I don't upload any video just I am clicking the upload button its not validating". This is my code, what editing I have to do?

Sample.aspx

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>

 $(function () {
    $('#<%=FileUploadbtn.ClientID %>').change(
             function () {
                 var fileExtension = ['mp4'];
                 if ($.inArray($(this).val().split('.').pop().toLowerCase(), fileExtension) == -1) {
                     // alert("Invalid");
                     $('#<%=uploadbtn.ClientID %>').attr("disabled", true);
                     $('#<%= lblError.ClientID %>').html("Invalid");
                 }
                 else {
                     $('#<%=uploadbtn.ClientID %>').attr("disabled", false);
                     $('#<%= lblError.ClientID %>').html(" ");
                 }
             })
})

ASPX Code

<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="Button1" runat="server" Text="Upload" CssClass="button" OnClick="uploadbtn_Click" />
<asp:Label ID="lblError" runat="server" Text="Label"></asp:Label>