Edge rails, single table inheritance and keeping multiple classes in a single file

So is the official stance as per edge rails that multiple class
definitions must go into separate files? I was trying to use STI with
acts_as_attachment, to have all my attachment classes go into a single
file, for example:

attachment.rb

class Attachment < ActiveRecord::Base
end

class UserPicture < Attachment
belongs_to :user, :foreign_key => 'owner_id'
acts_as_attachment ...
end

but I get the error: superclass mismatch for class UserPicture

when using "model :attachment" in my controller.. This used to work
under rails 1.1.6. I found the following bug report regarding this
problem:

http://dev.rubyonrails.org/ticket/5775

Just wondering if this is something that's going to eventually be
fixed, or if from now on, we should start putting all separate classes
into individual files? Thanks,

Mike