rspec_test_file_upload.rb

require 'rails_helper'

describe DocumentsController do
  describe '#create' do
    it 'persists a file' do
      file_path = File.join(Rails.root, 'tmp', SecureRandom.uuid)
      File.open(file_path, 'w+b') { |f| f.write "A random file used for testing attachments!" }
      file = Rack::Test::UploadedFile.new(file_path, 'text/plain')
      post :create, {file: file}
      expect(Document.last.file).to eql 'A random file used for testing attachments!'
    end
  end
end