FileUpload Control Within An UpdatePanel

This will work by forcing a full postback. In order to do this, we use PostBackTrigger:

<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="conditional">
<Triggers>
<asp:PostBackTrigger ControlID="Button1" />
</Triggers>
<ContentTemplate>
<ews:DatePicker ID="DatePicker1" runat="server" UsingUpdatePanel="True" OnSelectionChanged="DatePicker1_SelectionChanged" /><br />
<asp:Label ID="Label1" runat="server"></asp:Label><br /><br />
<asp:FileUpload ID="FileUpload1" runat="server" /><asp:Button ID="Button1" runat="server"
Text="Upload" OnClick="Button1_Click" />
</ContentTemplate>
</asp:UpdatePanel>

See: Triggers - a better control over postback , FileUpload example