UpdatePanel doesnt work as intended for file upload control

When I click on the attach button, It adds a new row to the grid with the postedFile name. But its also causing the whole page to refresh, as if the updatePanel doesnt exist. Whereas, I would like just the gridview to update. Why does this have to be this way with file upload controls. The UpdatePanel works as desired for other controls/grids on page.

I do not have a knowledge on jquery, so have to work it out only with c#. Any suggestions.

<asp:updatepanel runat="server" updatemode="conditional">
 <triggers>
   <asp:postbacktrigger controlid="btnAttach"/>
 </triggers>
 <contenttemplate>
  <asp:gridview ...../>
  <asp:fileupload id="fup" runat="server">
  <asp:button id="btnAttach" text="attach" runat="server/>
 </contenttemplate>
</asp:updatepanel>