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: attachment.rb
    path of file: ./repos/brimir/app/models
    
    the code of the file until where you have to start completion: # Brimir is a helpdesk system to handle email support requests.
# Co
    
  def thumbnail?

    return false if disable_thumbnail_generation

    unless file_content_type.nil?

      if !file_content_type.match(/^image/).nil? &&
          system('which convert', out: '/dev/null')

        return true
      end