Don't require _content_type column, since sometimes you don't need it.

lib/paperclip.rb

@@ -113,7 +113,7 @@ module Paperclip
     def has_attached_file name, options = {}
       include InstanceMethods
 
-      %w(file_name content_type).each do |field|
+      %w(file_name).each do |field|
         unless column_names.include?("#{name}_#{field}")
           raise PaperclipError.new("#{self} model does not have required column '#{name}_#{field}'")
         end