Issue #1934568 by David_Rothstein, StryKaizer, borisson_, martin107, pwolanin, ofry, amitgoyal: Allow sites using the 'image_allow_insecure_derivatives' variable to have partial protection from the Drupal 7.20 security issue

Issue #2330899 by attiks, Jelle_S, fietserwin: Allow image effects to change the MIME type + extension, add a convert image effect.

Issue #2272481 by herom, ParisLiakos, tim.plunkett: Remove usages of watchdog() from forms, plugins and controllers.

Issue #2218463 by znerol, damiankloip, rbayliss: Fixed Private file and image style downloads throw 500 error when page cache is active.