I just solved this, pieced this together from other answers on SO.
file = StringIO.new(a1.decoded_content)
file.class.class_eval { attr_accessor :original_filename, :content_type}
file.original_filename = a1.name
file.content_type = a1.type
@d = Document.create #Document is my model that has a paperclip attachment called doc
@d.doc = file
@d.save
Good luck!
Ben