RequiredFieldValidator disappears when user clicks on upload button (On Postback)

see my below code



function setValidations() {
var selValue = document.getElementById("DDLID").value;
var reqVal = document.getElementById("RequiredFieldValidatorID");
var reqVal2 = document.getElementById("RequiredFieldValidatorID2");
if (selValue == "a") {
ValidatorEnable(reqVal, false);
ValidatorEnable(reqVal2, false);
}
else {
ValidatorEnable(reqVal, true);
ValidatorEnable(reqVal2, true);
}
}
</script>




<asp:DropDownList ID="DDLID" runat="server" onchange="setValidations()" ValidationGroup="abc">
<asp:ListItem Value="a">No
<asp:ListItem Value="b">Yes

<asp:TextBox ID="TextBox1" runat="server" >
<asp:RequiredFieldValidator ID="RequiredFieldValidatorID" runat="server"
ControlToValidate="TextBox1" ErrorMessage="*" ValidationGroup="abc" >
<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="Button2" runat="server" Text="upoad" OnClick="btnUpload_Click" />
<asp:RequiredFieldValidator ID="RequiredFieldValidatorID2" runat="server" ValidationGroup="abc"
ControlToValidate="FileUpload1" ErrorMessage="*" >
<asp:Button ID="Button1" runat="server" Text="save" OnClick="btnUpload_Click"/>