ruby-on-rails – 如何在不编写文件的情况下找出mime类型？

def create
    @person = Person.new(person_params)
    if @person.image
      cur_time_in_ms = DateTime.now.strftime('%Q')
      file_location = "/tmp/file#{cur_time_in_ms}.ext"
      File.binwrite(file_location,@person.image.read)
      fm = FileMagic.new(FileMagic::MAGIC_MIME)
      @person.content_type = fm.file(file_location,true)
    end
    if @person.save
      redirect_to @person
    else
      # This line overrides the default rendering behavior,which
      # would have been to render the "create" view.
      render "new"
    end
  end

困扰我这种方法的事情是,我必须在确定其mime类型之前将文件写入文件系统.这似乎很浪费.如何在不先创建文件的情况下找出mime类型？

编辑：为了回答给出的答案,我重新安排了一些事情,但是现在“content_type”变成了“application / x-empty”,即使我上传了一个有效的png文件.这是代码

if @person.image

  cur_time_in_ms = DateTime.now.strftime('%Q')
  file_location = "/tmp/file#{cur_time_in_ms}.ext"
  File.binwrite(file_location,@person.image.read)
  file = File.open(file_location,"rb")
  contents = file.read
  # Scale image appropriately
  #img = Magick::Image::read(file_location).first
  @person.content_type = FileMagic.new(FileMagic::MAGIC_MIME).buffer(@person.image.read,true)
  @person.image = contents