favicon.ico in the root of the packaged war file (from http://mirrors.jenkins.io/war/) is being mangled since Jenkins v2.79.

Compared to previous releases, the favicon file has doubled in size and now can no longer be read by browsers or by image editing software such as GIMP.

The favicon in the source on github hasn't changed in 7 years, so I would guess this is as a result of some kind of change in war packaging, such as an encoding issue.