Cocoon not saving all fields

_attachment_fields.html.erb

<div class="well">
<%= f.label :attachments %>
<%= f.file_field :picture %>
<%= link_to_remove_association 'Remove image', f, {wrapper_class: "well"} %>
</div>

_form.html.erb

<%= form_for(@post) 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 id="attachments">
        <%= f.fields_for :attachments do |at| %>
            <%= render 'attachment_fields', :f => at %>
            <%= link_to_add_association 'Add another image', f, :attachments %>
        <% end %>
</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

Post.rb

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

    has_and_belongs_to_many :categories, join_table: :categories_posts
    accepts_nested_attributes_for :categories

end

posts_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

  # GET /posts/1/edit
  def edit
  end

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

    respond_to do |format|
      if @post.save
        format.html { redirect_to @post, notice: 'Post was successfully created.' }
        format.json { render action: 'show', status: :created, location: @post }
      else
        format.html { render action: 'new' }
        format.json { render json: @post.errors, status: :unprocessable_entity }
      end
    end
end


private
  # Use callbacks to share common setup or constraints between actions.
    def set_post
      @post = Post.find(params[:id])
    end
    
 def post_params
      params.require(:post).permit(:title, :content, :excerpt, {:category_ids=>[]}, attachments_attributes:[:id, :picture, :_destroy])
    end