migration to set content types of all existing attachments

db/migrate/20140430125956_reset_content_types.rb

@@ -0,0 +1,11 @@
+class ResetContentTypes < ActiveRecord::Migration
+  def up
+    Attachment.all.each do |attachment|
+      attachment.update_column(:content_type, Attachment.content_type_for(attachment.diskfile))
+    end
+  end
+
+  def down
+    # noop
+  end
+end