Paperclip is a very handy ruby gem to deal with media uploaded to Amazon S3. I works fine with content uploaded from your Rails application, but sometimes you’ll want to use it as well for files already stored in Amazon S3. In that case, you’ll need to manually set the paperclip attributes. I’m using this apporach: Using Paperclip with files already on Amazon S3. Since your files are already in S3 you can get the attributes using the aws-sdk gem: Class: AWS::S3::S3Object.
def set_file_as_attachment
s3 = AWS::S3.new
obj = s3.buckets['your-bucket'].objects['path/you/file.mp4']
self.attachment_file_name = 'file.mp4'
self.attachment_content_type = MIME::Types.type_for(self.media_file_name).first.to_s
self.attachment_file_size = obj.content_length
self.attachment_updated_at = Time.now
self.save!
end