destroyを使います。
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
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
destroyはclear+saveだとリファレンスに書いてありますが、中身もそのまんまでした。
lib/paperclip/attachment.rb
# line 259
def destroy
clear
save
end