You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: test_attachment.rb
    path of file: ./repos/sendgrid-ruby/test/sendgrid/helpers/mail
    
    the code of the file until where you have to start completion: require_relative "../../../../lib/sendgrid/helpers/mail/attachment"
include SendGrid
require "json"
require "minitest/autorun"

class TestAttachment < Minitest::Test
  SAMPLE_INPUT = "Es blüht so grün wie Blüten blüh'n im Frühling
Es blüht so grün wie Blüten blüh'n im Frühling
Es blüht so grün wie Blüten blüh'n im Frühling
Es blüht so grün wie Blüten blüh'n im Frühling
Es blüht so grün wie Blüten blüh'n im Frühling
".force_encoding('UTF-8').encode

  def setup; end

  def test_io_enocding
    attachment = Attachment.new
    attachment.content = StringIO.new
    
  def setup; end

  def test_io_enocding
    attachment = Attachment.new
    attachment.content = StringIO.new(SAMPLE_INPUT)

    expected = {
      "content" => "RXMgYmzDvGh0IHNvIGdyw7xuIHdpZSBCbMO8dGVuIGJsw7xoJ24gaW0gRnLD\nvGhsaW5nCkVzIGJsw7xodCBzbyBncsO8biB3aWUgQmzDvHRlbiBibMO8aCdu\nIGltIEZyw7xobGluZwpFcyBibMO8aHQgc28gZ3LDvG4gd2llIEJsw7x0ZW4g\nYmzDvGgnbiBpbSBGcsO8aGxpbmcKRXMgYmzDvGh0IHNvIGdyw7xuIHdpZSBC\nbMO8dGVuIGJsw7xoJ24gaW0gRnLDvGhsaW5nCkVzIGJsw7xodCBzbyBncsO8\nbiB3aWUgQmzDvHRlbiBibMO8aCduIGltIEZyw7xobGluZwo=\n"
    }

    json = attachment.to_json

    # Double check that the decoded json matches original input.
    decoded = Base64.decode64(json["content"]).force_encoding('UTF-8').encode

    assert_equal(decoded, SAMPLE_INPUT)

    assert_equal(json, expected)
  end
end