The answer for your question is within your code
check for extension by string comparison
if(string.Compare(Path.GetExtension(file.FileName),".zip",true)!=0)
{
ShowMessage("You Can upload only zip files");
return;
}
void ShowMessage(msg)
{
ClientScript.RegisterStartupScript(GetType(),"msg","alert('"+ msg +"');",true)
}
and for fie size
if((file.FileBytes.LongLength/(1024*1024))>10)
{
ShowMessage("You can upload upto 10Mb");
}
--Pankaj