I see a lot of such queries in "Sending Data" state, using SHOW PROCESSLIST.
Context is a high traffic Wordpress powered news site.
SELECT post_id, meta_value FROM wp_postmeta WHERE `meta_key` = '_wp_attached_file' AND LOWER(`meta_value`) IN ('2016/06/file-1.jpg', 'A LONG LIST OF FILES HERE', ... ) LIMIT 76
So far, I have been unable to find what generates these queries. I tried searching for various parts of the query like '_wp_attached_file', 'post_meta' but found no places in the core where this would be generated, nor in my theme or plugins.