@klass is no longer used in rake task. obtain_class should return the class name.

lib/tasks/paperclip.rake

 def obtain_class
   class_name = ENV['CLASS'] || ENV['class']
   raise "Must specify CLASS" unless class_name
+  class_name
 end
 
-def obtain_attachments
+def obtain_attachments(klass)
+  klass = Object.const_get(klass.to_s)
   name = ENV['ATTACHMENT'] || ENV['attachment']
-  raise "Class #{@klass.name} has no attachments specified" unless @klass.respond_to?(:attachment_definitions)
-  if !name.blank? && @klass.attachment_definitions.keys.include?(name)
+  raise "Class #{klass.name} has no attachments specified" unless klass.respond_to?(:attachment_definitions)
+  if !name.blank? && klass.attachment_definitions.keys.include?(name)
     [ name ]
   else
-    @klass.attachment_definitions.keys
+    klass.attachment_definitions.keys
   end
 end
 
@@ -21,8 +23,8 @@ namespace :paperclip do
     desc "Regenerates thumbnails for a given CLASS (and optional ATTACHMENT)."
     task :thumbnails => :environment do
       errors = []
-      names = obtain_attachments
       klass = obtain_class
+      names = obtain_attachments(klass)
       names.each do |name|
         Paperclip.each_instance_with_attachment(klass, name) do |instance|
           result = instance.send(name).reprocess!
@@ -34,8 +36,8 @@ namespace :paperclip do
 
     desc "Regenerates content_type/size metadata for a given CLASS (and optional ATTACHMENT)."
     task :metadata => :environment do
-      names = obtain_attachments
       klass = obtain_class
+      names = obtain_attachments(klass)
       names.each do |name|
         Paperclip.each_instance_with_attachment(klass, name) do |instance|
           if file = instance.send(name).to_file
@@ -53,8 +55,8 @@ namespace :paperclip do
 
   desc "Cleans out invalid attachments. Useful after you've added new validations."
   task :clean => :environment do
-    names = obtain_attachments
     klass = obtain_class
+    names = obtain_attachments(klass)
     names.each do |name|
       Paperclip.each_instance_with_attachment(klass, name) do |instance|
         instance.send(name).send(:validate)