attachment.rb

class Attachment < ActiveRecord::Base
  mount_uploader :attachment, AttachmentUploader

  # Associations

  belongs_to :attached_item, polymorphic: true

  # Validations

  validates_presence_of :attachment
  validates_integrity_of :attachment

  # Callbacks

  before_save :update_attachment_attributes

  # Delegate

  delegate :url, :size, :path, to: :attachment

  # Virtual attributes

  alias_attribute :filename, :original_filename

  private
  
  def update_attachment_attributes
    if attachment.present? && attachment_changed?
      self.original_filename = attachment.file.original_filename
      self.content_type = attachment.file.content_type
    end
  end
end

attachment_uploader.rb

# encoding: utf-8

require 'carrierwave/processing/mime_types'

class AttachmentUploader < CarrierWave::Uploader::Base
  include CarrierWave::MimeTypes

  storage :file

  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  process :set_content_type
end

attachments_controller.rb

class AttachmentsController < ApplicationController
  before_action :find_attachable
  
  def create
    @attachable.attachments.create attachment_params
    redirect_to @attachable
  end
  
  private
  
  def attachment_params
    params.require(:attachment).permit(:attachment)
  end
  
  # could be improve and include into concerns
  # http://api.rubyonrails.org/classes/ActiveSupport/Concern.html
  def find_attachable
    params.each do |name, value|
      return @attachable = $1.classify.constantize.find(value) if name =~ /(.+)_id$/
    end
  end
end

new.html.erb

# Required gem ryanb/nested_form
# https://github.com/ryanb/nested_form

<%= nested_form_for @post, html: { multipart: true } do |f| %>
  <p><%= f.link_to_add "Add an attachment", :attachments %></p>
  
  <%= f.fields_for :attachments do |attachment_form| %>
    <%= attachment_form.file_field :attachment %>
    <%= attachment_form.hidden_field :attachment_cache %>
    <%= attachment_form.link_to_remove "Remove this attachment" %>
  <% end %>
<% end %>

post.rb

class Post < ActiveRecord::Base
  has_many :attachments, as: :attached_item, dependent: :destroy
  
  # http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html#method-i-accepts_nested_attributes_for
  accepts_nested_attributes_for :attachments, allow_destroy: true, reject_if: proc { |attributes| attributes[:attachment].nil? }
end

posts_controller.rb

class PostsController < ApplicationController
  def new
    @post = Post.new
  end
  
  def create
    @post = Post.create post_params
    redirect_to @post
  end
  
  private
  
  def post_params
    params.require(:post).permit(attachments_attributes: [:id, :attachment, :attachment_cache, :_destroy])
  end
end

route.rb

App::Application.routes.draw do
  # Concerns must be declared before
  # http://api.rubyonrails.org/classes/ActionDispatch/Routing/Mapper/Concerns.html
  concern :attachable do
    resources :attachments, only: :create
  end
  
  resources :posts, concerns: [:attachable]
end

schema.rb

create_table "attachments", force: true do |t|
  t.integer  "attached_item_id"
  t.string   "attached_item_type"
  t.string   "attachment",         null: false
  t.string   "original_filename"
  t.string   "content_type"
  t.datetime "created_at"
  t.datetime "updated_at"
end

add_index "attachments", ["attached_item_type", "attached_item_id"]