Make the paperclip filename interpolator smarter about the :original style

If an :original gets converted into another format, it would get saved as
original_filename *anyway*, so generating the extension is pointless and
yields bad results for when you change the style definition later. This way,
old gifs will still have correct URLs

config/initializers/paperclip.rb

@@ -2,6 +2,11 @@
 
 Paperclip.options[:read_timeout] = 60
 
+Paperclip.interpolates :filename do |attachment, style|
+  return attachment.original_filename if style == :original
+  [basename(attachment, style), extension(attachment, style)].delete_if(&:empty?).join('.')
+end
+
 if ENV['S3_ENABLED'] == 'true'
   Aws.eager_autoload!(services: %w(S3))