This post shows how to use carrierwave gem to upload multiple files in Rails 4

Let suppose the following scenario. We have multiple projects and each project can have multiple screenshots.

In project.rb

class Project < ActiveRecord::Base
   has_many :screenshots
   accepts_nested_attributes_for :screenshots, :allow_destroy => true
end

In screenshot.rb

class Screenshot < ActiveRecord::Base
  belongs_to :project
  mount_uploader :image, ScreenshotUploader
end

In project_controller.rb

class ProjectsController < ApplicationController
 
  # GET /projects/new
  def new
    @project = Project.new
    @project.screenshots.new
  end

  # GET /projects/1/edit
  def edit    
      @project.screenshots.new   
  end

  # POST /projects
  # POST /projects.json
  def create
    @project = Project.new(project_params)

    respond_to do |format|
      if @project.save
        if params[:screenshots_attributes]
          params[:screenshots_attributes].each do |screenshot|
            @project.screenshots.create(image: screenshot[:image])
          end
        end
        format.html { redirect_to @project, notice: 'Project was successfully created.' }
        format.json { render :show, status: :created, location: @project }
      else
        format.html { render :new }
        format.json { render json: @project.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /projects/1
  # PATCH/PUT /projects/1.json
  def update
    respond_to do |format|
      if @project.update(project_params)
        if params[:screenshots_attributes]
          params[:screenshots_attributes].each do |screenshot|
            @project.screenshots.create(image: screenshot[:image])
          end
        end
        format.html { redirect_to @project, notice: 'Project was successfully updated.' }
        format.json { render :show, status: :ok, location: @project }
      else
        format.html { render :edit }
        format.json { render json: @project.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /projects/1
  # DELETE /projects/1.json
  def destroy
    @project.destroy
    respond_to do |format|
      format.html { redirect_to projects_url, notice: 'Project was successfully destroyed.' }
      format.json { head :no_content }
    end
  end

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

    # Never trust parameters from the scary internet, only allow the white list through.
    def project_params
      params.require(:project).permit(:title, :description, :screenshots_attributes => [:image])
    end
end

In views/projects/_forms.html.erb

<%= form_for @project, :html => {:multipart => true,  :class => "form-horizontal project" } do |f| %>

  <div class="form-group">
    <%= f.label :title, :class => 'col-sm-2 control-label' %>
    <div class="col-sm-10">
      <%= f.text_field :title, :class => 'form-control' %>
    </div>
    <%= error_span(@project[:title]) %>
  </div>
  <div class="form-group">
    <%= f.label :description, :class => 'col-sm-2 control-label' %>
    <div class="col-sm-10">
      <%= f.text_area :description, :class => 'form-control' %>
    </div>
    <%= error_span(@project[:description]) %>
  </div>
 
  <div class="form-group">
    <%= f.fields_for :screenshots, @project.screenshots do |f_screenshot| %>
      <%= f_screenshot.label :image, :class => 'col-sm-2 control-label' do %>
        Screenshots
      <% end %>
      <div class="col-sm-10">
        <%= f_screenshot.file_field :image, multiple: true, :name=>'project[screenshots_attributes][][image]', :class => 'form-control' %>
      </div>
    <% end %>
  </div> 

  <div class="form-actions">
      <%= f.submit nil, :class => 'btn btn-primary' %>
      <%= link_to t('.cancel', :default => t("helpers.links.cancel")),
                projects_path, :class => 'btn btn-default' %>
  </div>

<% end %>

Reference: Stackoverflow