One of my jobs produces plain-text files that are in UTF-8. When I view them at /job/jobname/ws/filename.txt, I see garbage because Jenkins serves them with

Content-Type: text/plain;charset=ISO-8859-1

Jenkins's /systemInfo says

file.encoding: UTF-8
sun.jnu.encoding: UTF-8

and environment variables contain

LANG en_US.UTF-8

so I've no idea why Jenkins thinks it should use Latin-1 instead.