Quick and dirty one off migration task for paperclip plugin to move files from local filesystem to Amazon S3.

paperclip_migration.rake

# First configure your models to use Amazon s3 as storage option and setup the associated S3 config.
# Then add the classes your want to migrate in the klasses array below.
# Then run rake paperclip_migration:migrate_to_s3
# Should work but this is untested and may need some tweaking - but it did the job for me.

namespace :paperclip_migration do  
  desc "migrate files from filesystem to s3"
  task :migrate_to_s3 => :environment do
    klasses = [:model_1, :model_2] # Replace with your real model names. If anyone wants to this could be picked up from args or from configuration.
    klasses.each do |klass_key|
      if klass = real_klass(klass_key)
        if klass.respond_to?(:attachment_definitions) && definitions = klass.attachment_definitions
          klass.all.each do |record|
            definitions.keys.each do |definition|
              if record.send("#{definition}_file_name")
                attachment = Paperclip::Attachment.new(definition.to_sym, record, definitions[definition.to_sym].except(:s3_credentials, :storage))
                if File.exists?(Rails.root.to_s + "/public" + attachment.url)
                  file_data =  File.open(Rails.root.to_s + "/public" + attachment.url)
                  puts "Saving file: #{Rails.root.to_s + "/public" + attachment.url} to Amazon S3..."
                  record.send("#{definition}").assign file_data
                  record.send("#{definition}").save
                else
                  puts "Can't find file: #{Rails.root.to_s + "/public" + attachment.url} NOT MIGRATING..."
                end
              end
            end
          end
        else
          puts "There are not paperclip attachments defined for the class #{klass.to_s}"
        end
      else
        puts "#{key.to_s.classify} is not defined in this app."
      end
    end
  end
  
  def real_klass(key)
    key.to_s.classify.constantize
  rescue
  end
end

Appreciate it!

Another hint: Since attachment.url automatically attaches the timestamp to the url in new Paperclips, the file will not be found. If you pass some arguments to the url method on the attachment you can fix this. See my forked Gist.