Ariana

This is a very simple plugin that adds a “robots” meta tag to the head of your site with “noindex” in it.
The effect of the noindex robots meta tag is that search engines will not index your pages and posts at all. 
This is a better solution then the wordpress default settings option, which tells search engines to not crawl the URLs in the robots.txt. Because Google (and other search engines) sometimes still index URLs they haven’t crawled, for which they determine relevancy based on anchor text of links which lead to the specific URL.

Simple Noindex

modernwebstudios

This plugin add meta tag noindex,nofolow to head of any attachment page().

MWS Noindex Attachment Pages

This plugin add meta tag noindex,nofolow to head of any attachment page().

Simple Noindex vs MWS Noindex Attachment Pages