Oliver Gondža just rejected it a few days ago for 1.596.3, I am guessing because it is too new and did not have the requisite two-week soaking period. That would mean it would have to wait till the next line, something like 1.608.1 (TBD). Bring up on jenkinsci-dev if you feel this needs to be in 1.596.3.
(Do you say that this was a regression from an earlier Jenkins release? My impression was that it was always this way, and Chrome just recently started enforcing the content type. Issues with the regression label deserve special consideration.)
FYI, *.ci.cloudbees.com are already running this patch.