The download from update.jenkins-ci.org has some non-JSON in the first and last lines (why!?). Perhaps Jenkins is ignoring your new file because of this...? You can strip them out:
curl -L http://updates.jenkins-ci.org/update-center.json | sed '1d;$d' > default.json (paraphrased from here: https://gist.github.com/jedi4ever/898114) -- You received this message because you are subscribed to the Google Groups "Jenkins Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/jenkinsci-users/4eb1d0ee-84c1-477f-bdb8-9e4d0067e6ec%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
