A simple rack app for educational purposes: upload a file and have it presented in the browser

config.ru

class App
  def self.call env
    new(env).call
  end

  def initialize env
    @request = Rack::Request.new env
    @response = Rack::Response.new
  end

  def set_response
    case @request.request_method
    when 'GET'
      @response.set_header 'Content-Type', 'text/html'
      @response.write '<form method="post" enctype="multipart/form-data">'
      @response.write '<input type="file" name="myfile"></input><input type="submit"></input>'
    when 'POST'
      @response.set_header 'Content-Type', Rack::Mime.mime_type(File.extname @request.params['myfile'][:tempfile])
      @response.write @request.params['myfile'][:tempfile].read
    else
      raise 'Method unknown!'
    end
  end

  def call
    set_response
    @response.finish
  end
end

run App