Issue #2551373 by drpal, danlinn, ruthleopold, Berdir, droplet, Wim Leers, prajaankit: contextual.js and quickedit.js should fail gracefully, with useful error messages, when Twig templates forget to print attributes

Issue #625958 by slashrsm, quicksketch, eojthebrave, droplet, danlinn, czigor: Support Uploading Multiple Files for HTML5 Browsers via #multiple attribute.