Normally I'd use carrierwave, but they do not officially support jruby, and I've been running in to bugs, possibly related. (https://github.com/jnicklas/carrierwave/issues/620, Image corruption on upload to s3, production only. (carrierwave, engineyard))
Have others had success here?
I'm considering trying out paperclip, but it looks like that may not be perfect either-- https://github.com/thoughtbot/paperclip/issues/100