Feature test error - uninitialized constant FFaker::File

This is my feature spec test for creating a new post

require 'rails_helper'
require 'ffaker'

RSpec.feature 'Creating posts', :type => :feature do
  scenario 'can create a new post' do
    visit root_path
    click_link 'New Post'
    attach_file('Image', FFaker::File.file_name('spec/files', 'foo', 'jpg'))
    fill_in 'Caption', with: 'Hello World! This is the first post!'
    click_button 'Create Post'
    expect(page).to have_content('Post was successfully created')
  end
end

When I run the rspec command to run the test I get the following error

Failure/Error: attach_file('Image', FFaker::File.file_name('spec/files', 'foo', 'jpg'))

NameError:
       uninitialized constant FFaker::File

How can I fix this?

Basically I htought to use ffaker for trying to upload an image instead of using a real image file. Is this supposed to work with ffaker, right?