Katello devs seeing 'uninitialized constant Katello::Concerns::MediumExtensions'

If you're seeing:

katello/lib/katello/engine.rb:175:in `block in ': uninitialized constant
Katello::Concerns::MediumExtensions (NameError)

when starting the rails server or console, you may need to restart
spring. It apparently can't handle reloading of engine.rb properly,.
Make sure you are on the latest master and run:

spring stop

Then continue as normal, spring will restart.