Why would this effect a Boro attendance? I mean we've had no domestic football for weeks. I believe any Boro fan would prefer to see Boro no matter what stage of a tournament England are at.
Apart from that it's an evening kick off and doesn't even clash. It's actually ideal as you can watch the Boro and straight down the pig iron for the England game

I'd be watching the England match if they kicked off at the same.
I know it's a minority opinion on here, but I'm country before club.
I love both, but if I had to choose, it would be England, unless it was a friendly/qualifier/dead rubber.
Especially now you can watch the Boro matches online easily.
Again though, Senegal will be a very tough opponent, there's no point in counting chickens yet.