When a file is selected using FileUpload Control ,then on postback, PostedFile property gets initialized with HttpPostedFile object for the file. Since http request cannot maintain state, so it looses it's state.
use update panel before file upload control.
like:-
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:FileUpload ID="fileUploadImage" runat="server"></asp:FileUpload>
<asp:Button ID="btnUpload" runat="server" Text="Upload Image"
OnClick="btnUpload_Click" />
</ContentTemplate>
<Triggers>
<asp:PostBackTrigger ControlID="btnUpload" />
</Triggers>
</asp:UpdatePanel>