What to do with them? In our major Yoast SEO 7.0 update, there was a bug concerning attachment URL’s. We quickly resolved the bug, but some people have suffered anyhow (because they updated before our patch). This post serves both as a warning and an apology. We want to ask all of you to check whether your settings for the redirect of the attachment URL’s are correct. And, for those of you who suffered from a decrease in rankings because of incorrect settings, we offer a solution that Google has OKed as well.
Is redirect attachment URLs set to “Yes”?
You need to check this manually: unless you have a very specific reason to allow attachment URLs to exist (more on that below), the setting should be set to “Yes” . If the setting says “Yes”, you’re all set. You can find this setting in Search Appearance, in the tab Media.
Is your attachment URL set to “No”?
If your attachment URL is set to “no”, there are two different scenario’s which could apply to you. You could intentionally have set this setting to “no”, but the setting could also be turned to “no” without your intent.
Intentionally
Source: https://managewp.org/articles/17488/yoast-attachment-url-bug-and-rankings-loss-solution
Willie Chiu's Blog 