title: Rails active storage has_many_attachments permit params issue
body: <p>I want to upload multiple images in form by using active storage but it gives an permit params issue <code>Unpermitted parameter: :avatars</code></p>
  
  <pre><code>params.require(:poi).permit(:title, :description,avatars: [])
  </code></pre>
  
  <p>In <code>_form.html.erb</code>,</p>
  
  <pre><code>&lt;%= form.file_field :avatars %&gt;
  </code></pre>
  
  <p>In <code>poi.rb</code>,</p>
  
  <pre><code>has_many_attached :avatars
  </code></pre>
  
  <p>In controller,</p>
  
  <pre><code>@poi = Poi.new(poi_params)
  
  respond_to do |format|
    if @poi.save
      format.html { redirect_to @poi, notice: 'Poi was successfully created.' }
      format.json { render :show, status: :created, location: @poi }
    else
      format.html { render :new }
      format.json { render json: @poi.errors, status: :unprocessable_entity }
    end
  end
  </code></pre>
  
  <p>How can I fix this issue?</p>