Use Ajax update panel..
The Example Below will be useful for you...
Bcos I got the same problem while using FileUpload control.
Example:
<table>
<tr>
<td>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="Button1" runat="server"
Text="Save" onclick="Button1_Click1" />
</ContentTemplate>
<Triggers>
<asp:PostBackTrigger ControlID="Button1" />
</Triggers>
</asp:UpdatePanel>
</td>
</tr>
</table>
</fieldset>
</div>
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