Paperclipのattachmentの削除方法 ref: http://qiita.com/jacoyutorius/items/3b597944363ba7fccf03

attachment.rb

# line 259
def destroy
  clear
  save
end

file0.txt

class Picture < ActiveRecord::Base
  [:picture].each do |col|
    has_attached_file col,
    styles: {:medium => "300x300", :thumb => "100x100"},
    default_url: "common/:style/icon-file-nodata.png",
      url: "/data/images/:class/:parent_id/:id/:attachment/:style/:filename"
    validates_attachment_content_type col,
      :content_type => ["image/png", "image/jpg", "image/jpeg"]
  end
end

file1.txt

Picture.first
Picture.first.picture
#=> Paperclip::Attachment
Picture.first.picture.destroy
#=> 
[paperclip] deleting ~/pictures/1/pictures/original/5558.jpg
[paperclip] deleting ~/pictures/1/pictures/medium/5558.jpg
[paperclip] deleting ~/pictures/1/pictures/thumb/5558.jpg
[paperclip] deleting ~/pictures/1/pictures/large/5558.jpg
=> true