test_rails_format_accept.rb

# frozen_string_literal: true

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'rails', '5.1.7'
end

require 'rack/test'
require 'action_controller/railtie'

class TestApp < Rails::Application
  config.root = __dir__
  config.session_store :cookie_store, key: 'cookie_store_key'
  secrets.secret_key_base = 'secret_key_base'

  config.logger = ::Logger.new($stdout)
  ::Rails.logger = config.logger

  routes.draw do
    post '/' => 'test#create'
  end
end

class TestController < ActionController::Base
  include ::Rails.application.routes.url_helpers

  def create
    respond_to do |format|
      format.json do
        render json: { messsage: 'success' }
      end

      format.html do
        raise 'Unreachable'
      end
    end
  end
end

require 'minitest/autorun'

class BugTest < Minitest::Test
  include ::Rack::Test::Methods

  def setup
    FileUtils.touch('dummy.txt')
  end

  def teardown
    FileUtils.remove('dummy.txt')
  end

  def test_returns_success
    post(
      '/',
      {
        file: Rack::Test::UploadedFile.new('dummy.txt')
      },
      'HTTP_ACCEPT' => 'application/json'
    )
    assert_equal('application/json', last_response.media_type)
  end

  private

  def app
    ::Rails.application
  end
end