The opening of the 12th Tribeca Film Festival, which was launched to revitalize Lower Manhattan in the wake of 9/11, took a poignant tone Wednesday after the Boston attack. Emcee Richard Belzer proclaimed, “Tonight we are going to dedicate this festival to the people of Boston . . . Tonight, we are all Bostonians.” After a screening of opening movie, “Mistaken for Strangers,” a doc on Brooklyn band the National, guests including Josh Radnor and Paul Haggis headed to the Highline Ballroom to see them play. “I’m taking Justin Bieber down!” singer Matt Berninger joked to us of his band’s burst of Tribeca popularity. “It’s funny to look at the charts and say, ‘Oh, I have the same job as Rihanna.’ ”