Use map,
map $request $allowonly {
default 1;
~*addmedia\.upload([a-zA-Z0-9-_=&]) 0;
1;
}
inside location {} if ($allowonly) { return 404; }
Untested but should give you enough to test with.
Posted at Nginx Forum: http://forum.nginx.org/read.php?2,256336,256337#msg-256337