Decrease Memory footprint

Mime-types 2.6.1+ has a columnar store that only loads the data that is
needed for a drastically reduced memory footprint.
cc/ @jeremyevans @halostatue

See: https://github.com/mime-types/ruby-mime-types/pull/96.

[fixes #1873]

features/step_definitions/attachment_steps.rb

@@ -49,7 +49,13 @@ end
 
 Then /^the attachment should have the same content type as the fixture "([^"]*)"$/ do |filename|
   in_current_dir do
-    require 'mime/types'
+    begin
+      # Use mime/types/columnar if available, for reduced memory usage
+      require "mime/types/columnar"
+    rescue LoadError
+      require "mime/types"
+    end
+
     attachment_content_type = `bundle exec #{runner_command} "puts User.last.attachment_content_type"`.strip
     attachment_content_type.should == MIME::Types.type_for(filename).first.content_type
   end

lib/paperclip.rb

@@ -56,7 +56,14 @@ require 'paperclip/has_attached_file'
 require 'paperclip/attachment_registry'
 require 'paperclip/filename_cleaner'
 require 'paperclip/rails_environment'
-require 'mime/types'
+
+begin
+  # Use mime/types/columnar if available, for reduced memory usage
+  require "mime/types/columnar"
+rescue LoadError
+  require "mime/types"
+end
+
 require 'mimemagic'
 require 'mimemagic/overlay'
 require 'logger'