config-routes.rb

SuperSecret::Application.routes.draw do
  # ...
  resources :magick_file_uploads, only: [:new, :create] do
    member do
      get :validate_status
    end
  end
  # ...
end

magick_file_uploads_controller.rb

class MagickFileUploadsController < ApplicationController
  expose(:magick_files){ current_organization.magick_files }
  expose(:magick_file)
  expose(:magick_file_upload)
  expose(:file_headers) { magick_file.magick_file_upload.file_headers }
  expose(:file_headers_minus_sku) { magick_mapping.magick_file_upload.file_headers_minus_sku }
  expose(:validation_data){
    data = magick_file_upload.generate_validation_data(
      validate_status_magick_file_magick_file_upload_url(magick_file, magick_file_upload)
    )
    sku_mapping_data = magick_file.generate_validation_mapping_data(
      validate_mapping_status_magick_file_url(magick_file)
    )
    data[:details]["sku"] = sku_mapping_data[:details]["sku"]
    data
  }
  expose(:validation_submit_path)   { magick_file_path(magick_file) }
  expose(:validation_back_path)     { magick_files_path }
  expose(:validation_forward_path)  { magick_files_path }

  before_filter { authorize! :manage, magick_file }
  skip_filter :require_initial_catalog!

  def create
    magick_file_upload = magick_file.magick_file_uploads.new(params[:magick_file_upload])
    magick_file_upload.magick_file = magick_file
    if params[:remote_information]
      magick_file.build_remote_information(params[:remote_information]).save
    end
    if magick_file_upload.save
      if params[:reminder]
        params[:reminder][:start_date] = DateTime.now+1.day
        params[:reminder][:email] = current_admin_user.email
        magick_file.build_reminder(params[:reminder]).save
      end
      redirect_to "/magick_files/#{magick_file.id}/magick_file_uploads/#{magick_file.magick_file_upload.id}/validate"
    else
      render :new
    end
  end

  def validate_status
    render json: magick_file_upload.allow_reupload(validation_data)
  end
end

https://github.com/hashrocket/decent_exposure