Last week I came across this bug (?) with AsyncFileUpload control in updatepanel. The control is not firing OnUploadedComplete server side event when it’s in updatepanel.
After doing some research on this issue, came across these solutions.
1) Add another AsyncFileUpload control on the page and set its’ style to display:none;
<div style="display: none;">
<cc1:AsyncFileUpload ID="AsyncFileUpload1" runat="server" />
2) Another workaround is to add the following attributes to the <form> tag of the page..
Both the solutions are working fine.
Thanks to obout_teo and MikeMelendez of Asp.Net forums