Multiple file upload for Rails 4 params

image.rb

class Image < ActiveRecord::Base
  belongs_to :post
  has_attached_file :file, :styles => { :medium => "300x300>", :thumb => "100x100>" }, :default_url => "/images/:style/missing.png"
  validates_attachment_content_type :file, :content_type => /\Aimage\/.*\Z/
end

new.html.erb

<%= form_for @post, :url => posts_path,:html => { :multipart => true } do |f| %>
  <% if @post.errors.any? %>
        <div class="sectionwrapper errorbox">
          <div id="errorExplanation">
            <div class="title"><%= pluralize(@post.errors.count, "error") %> prohibited this post from being saved:
            </div>
            <ul>
              <% @post.errors.full_messages.each do |msg| %>
                  <li><%= msg %></li>
              <% end %>
            </ul>
          </div>
        </div>
    <% end %>
    
    <div class="sectionwrapper">
      <%= f.label "Add pictures" ,:class =>"inputlabel" %>
      <%= f.fields_for :images do |i| %>
          <% if i.object.new_record? %>
          
              <%= i.file_field :file , :multiple => true%>
          <% else %>
              <div class="imagescontainer sectionwrapper">
                <% @post.images.each do |image| %>
                    <%= image_tag image.file.url %>
                <% end %>
              </div>

          <% end %>
      <% end %>
    </div>

    <div class="mid submitwrapper">
      <%= f.submit "" %>
    </div>
<% end %>

post.rb

class Post < ActiveRecord::Base
  #after_update :save_images
  default_scope {order('created_at DESC')}

  belongs_to :user
  has_many :images
  accepts_nested_attributes_for :images, allow_destroy: true

  validates :post_content, presence: true

  def save_images
    images.each do |image|
      image.save()
    end
  end
end

posts_controller.rb

class PostsController < ApplicationController
  layout "posts"

=begin
Default RUD code being generated. Not shown here as I'm too lazy to rewrite production code and remove sections.
=end

# POST /posts
  # POST /posts.json
  def create
    # Save all images
    @post = Post.new
    @post.user = current_user
    respond_to do |format|
      if @post.save
        if(post_params[:images_attributes])
          post_params[:images_attributes]["0"]["file"].each do |i|
            image = Image.new
            image.file = i
            image.post = @post
            image.save()
          end
        end

        format.html { redirect_to root_path, 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

    # Never trust parameters from the scary internet, only allow the white list through.
    def post_params
      params.require(:post).permit(:post_content, :test_result_cholera_o1, :test_result_cholera_o139,
                                   :test_result_nitrite, :test_result_coliform, :test_result_arsenic,
                                   :location,
                                   # This line turns out to be key to make rails 4 params work for mutliple files.
                                   images_attributes: [:file => []] )
    end
end

I've created this as I've spend almost 2 hours trying to figure out how to do multiple file upload with Rail 4 using params.

The problem here is on line 47 of posts_controller.rb. You need to properly specify this to match a file field that allows selection of multiple files for upload. All the rest is easy, just process all the pictures and save.

Note this is a naive approach for image processing as you should not block response while waiting for image processing to complete. You should setup resque to do the processing in background while you return success to the user.