How To Dispaly Confirm Box On Code Behind

Hi All
 
I need to dispay confirm box on server Side.. senario is like Im Trying To upload Image During Upload Im Checking some properties like Image type,image size, etc.. While Uploading i want to check properties like

if(type=="xxxx")
 
{
 
confirmbox("  ");
 
if(confirmvalue=="yes")
 
{
 
next condition
 
}
 
else
 
{
 
exit ;
 
}
 
}
 
like this i want check different properies of file.. i tried
 
this.Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('You clicked YES!')", true);

but it wont work