Added a callback compatability shim for older versions of Rails

lib/paperclip.rb

@@ -83,6 +83,7 @@ module Paperclip
 
     def included base #:nodoc:
       base.extend ClassMethods
+      base.send(:include, Paperclip::CallbackCompatability) unless base.respond_to?(:define_callbacks)
     end
 
     def processor name

lib/paperclip/callback_compatability.rb

+module Paperclip
+  # This module is intended as a compatability shim for the differences in callbacks
+  # between Rails 2.0 and Rails 2.1.
+  module CallbackCompatability
+    def self.included(base)
+      base.extend(ClassMethods)
+      base.send(:include, InstanceMethods)
+    end
+
+    module ClassMethods
+      # The implementation of this method is taken from the Rails 1.2.6 source,
+      # from rails/activerecord/lib/active_record/callbacks.rb, line 192.
+      def define_callbacks(*args)
+        args.each do |method|
+          self.class_eval <<-"end_eval"
+            def self.#{method}(*callbacks, &block)
+              callbacks << block if block_given?
+              write_inheritable_array(#{method.to_sym.inspect}, callbacks)
+            end
+          end_eval
+        end
+      end
+    end
+
+    module InstanceMethods
+      # The callbacks in < 2.1 don't worry about the extra options or the
+      # block, so just run what we have available.
+      def run_callbacks(meth, opts = nil, &blk)
+        callback(meth)
+      end
+    end
+  end
+end