jnicklas/carrierwave — pull request #669 by pupeno (2012-03-24 10:56:35+00:00)
Title: Generate a nicer validation error message by turning the extension white list into a sentence.

I monkey-patched this change in my Rails project and I think carrierwave might benefit from it as well. It might break tests as I haven't checked out carrierwave (yay for githubs editor). If you find it acceptable, then I can do the rest of the work of making sure tests pass, checking whether the white list actually responds to to_sentence, etc.