minitest_file_upload.rb

require 'test_helper'

class DocumentsControllerTest < ActionController::TestCase
  test "Uploading 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}
    assert_response :success
    assert_equal(Document.last.file, 'A random file used for testing attachments!') 
  end
end