dropzone + paperclip multiple image upload issue.

_form.html.erb

<%= form_for(@post, html: {class:"dropzone"}) do |f| %>
  <% if @post.errors.any? %>
    <div class="error_explanation">
      <h2><%= pluralize(@post.errors.count, "error") %> prohibited this post from being saved:</h2>

      <ul>
      <% @post.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

   <p>
        <%= f.label :title %> <br/>
        <%= f.text_field(:title, :size => "70") %>
    </p>

    <p class="well">
   <%= f.label :category %><br/>
    <%= hidden_field_tag "post[category_ids][]", nil%>

        <% Category.order(:title).each do |cat| %>
            <label>
            <%= check_box_tag("post[category_ids][]", cat.id, cat.id.in?(@post.categories.collect(&:id))) %>
            <%= cat.title %>
            </label>
        <% end %>
    </p>

    <p>
       <%= f.label :content %> <br/>
       <%= f.text_area(:content, :size => "90x12") %>

       <div class="fallback">
        <%= f.label :attachments %>
        <%= f.fields_for :attachments  do |at| %>
            <%= at.file_field :picture %>
        <% end %>

       </div>

        <%= render "shared/markdownhelp" %>

 </p>

    <p>
        <%= f.label :excerpt %><br/>
        <%= f.text_area(:excerpt, :size => "90x8") %>
    </p>


    <p>
        <%= f.submit %>
    </p>
<% end %>

attachment.rb

class Attachment < ActiveRecord::Base
  include Rails.application.routes.url_helpers

  belongs_to :imageable
  has_attached_file :picture,
                    :styles =>{ :medium => "660x",
                                :thumb => "150x"
                    },
                    :url => "/attachments/pictures/:style/:basename.:extension",
                    :path =>"#{Rails.root}/public/attachments/pictures/:style/:basename.:extension"
  validates_attachment_content_type :picture, :content_type => /\Aimage\/.*\Z/

end

params hash

post"=>{"title"=>"", "category_ids"=>[""], "content"=>"", "excerpt"=>"", "attachments_attributes"=>[{"picture"=>#<ActionDispatch::Http::UploadedFile:0x00000101a5c240 @tempfile=#<Tempfile:/tmp/RackMultipart20140729-45451-f62ok4>, @original_filename="IMG_0897.JPG", @content_type="image/jpeg", @headers="Content-Disposition: form-data; name=\"post[attachments_attributes][][picture]\"; filename=\"IMG_0897.JPG\"\r\nContent-Type: image/jpeg\r\n">}]}, "commit"=>"Create Post"}

post.rb

class Post < ActiveRecord::Base
    has_many :attachments, as: :imageable, dependent: :destroy
    accepts_nested_attributes_for :attachments

    has_and_belongs_to_many :categories, join_table: :categories_posts
    accepts_nested_attributes_for :categories

end

post_controller.rb

class PostsController < ApplicationController
  before_action :set_post, only: [:show, :edit, :update, :destroy]
  layout "admin"

  
  # GET /posts/new
  def new
    @post = Post.new
    @post.attachments.build
  end

 
  # POST /posts
  # POST /posts.json
  def create
    @post = Post.new(post_params)

    if @post.save
      render json: { message: "success"}, :status => 200
    else
      #  you need to send an error header, otherwise Dropzone
          #  will not interpret the response as an error:
      render json: { error: @post.errors.full_messages.join(',')}, :status => 400
    end

  end

 

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_post
      @post = Post.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def post_params
      params.require(:post).permit(:title, :content, :excerpt, {:category_ids=>[]}, attachments_attributes:[:picture])
    end
end

posts.js

$(document).ready(function(){
  // disable auto discover
  Dropzone.autoDiscover = false;

  // grap our upload form by its id
  $("#new_post").dropzone({
    // restrict image size to a maximum 1MB
    maxFilesize: 1,
    // changed the passed param to one accepted by
    // our rails app
    paramName: "post[attachments_attributes][][picture]",
    // show remove links on each image upload
    addRemoveLinks: true
  });
});