def file_field_sub(col_name, file_info, options)
      fields_for("file_info[#{col_name}]", file_info) do |f|
        f.file_field(:uploaded_data, {:size=>20}.merge(options))
      end
    end
def file_field_sub(col_name, file_info, options)
      fields_for("file_info[#{col_name}]", file_info) do |f|
        f.file_field(:uploaded_data, {:size=>20}.merge(options))
      end
    end
generate following file field:

   <input type='file' name='file_info[file_NN][uploaded_data]' ...>