_form.rb

<%= simple_form_for @item do |i| %>
    <%= i.input :name %>
    <%= i.input :description %>
    <%= i.input :price %>
    <%= i.input :zip %> 
    <%= i.input :weight %>
    <%= i.input :volume %>
    <%= i.simple_fields_for :item_images do |image| %>
        <% image.input :image, as: :file %>
    <% end %>
    <%= i.submit 'Submit', :class => 'button submit' %>
<% end %>

error

Unpermitted parameters: item_images

item_controller.rb

class ItemsController < ApplicationController
    def update
        @item = current_account.items.find(params[:id])
        if @item.update(item_params)
            render :edit
        else
            render :edit
        end
    end

  def item_params
    params.require(:item).permit(:name, :description, :price, :zip, :volume, :weight, item_images: {})
  end
end

params.json

  Parameters: {"utf8"=>"✓", "authenticity_token"=>"6oWxcCd4mbuZUQwdq3CXlNNLxrEUnjyP78kEU4OOwFQ=", "item"=>{"name"=>"arstarst", "description"=>"tsra", "price"=>"1.00", "zip"=>"123", "weight"=>"321.00", "volume"=>"123.00", "item_images"=>{"image"=>#
<ActionDispatch::Http::UploadedFile:0x007fd0c4d3f580 @tempfile=#<Tempfile:/var/folders/f5/ygvtnd1j25n5rkt1xqmmrsnm0000gn/T/RackMultipart20131112-65440-2ff3nc>, @original_filename="cute.jpg", @content_type="image/jpeg", @headers="Content-Disposition: form-data; name=\"item
[item_images][image]\"; filename=\"cute.jpg\"\r\nContent-Type: image/jpeg\r\n">}}, "commit"=>"Submit", "subdomain"=>"connect", "id"=>"7a87e70b-e04a-4f31-b5b4-c24595aeaf20"}