There have been a lot of mime-types added since 1.25.1, and there will be no updates at all to 1.x after October 27th, 2015. I also expect to release 3.0 later this year (absolutely no later than November 15th).
Because sup requires Ruby 2.0 or higher, you have no incompatibilities, and the mail
gem is already using mime-types 2.x (and I expect to release another version soon that will enable lazy loading to reduce the memory usage by default).