uploader.rb

class BaseUploader < CarrierWave::Uploader::Base
  include CarrierWave::MiniMagick

  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  def extension_white_list
    %w(jpg jpeg gif png)
  end
end

uploader_spec.rb

require 'rails_helper'

RSpec.describe BaseUploader do
  let(:model) { double "model", id: 1 }
  let(:uploader) { BaseUploader.new(model, :file) }

  before do
    uploader.store!(File.open("#{Rails.root}/spec/support/fixtures/file_name.png"))
  end

  after do
    uploader.remove!
  end

  it 'uploads file' do
    expect(uploader.url).to eq("/uploads/r_spec/mocks/double/file/1/file_name.png")
  end

  it 'allows just image upload' do
    expect(uploader.extension_white_list).to eq(%w(jpg jpeg gif png))
  end
end