This plugin is modded. It attaches unattached images to the correct post.

How to run it:

Tools > Media tools


At   "Choose tool to run",  select:

Import External Images

choose "Posts".

Perhaps it is better to test it first on a Date range than to scan your whole site. Or, perhaps set a few posts as Uncategorized and scan that cat only.

click "Import External Images"



NB:  The plugin will run, and give you a message like  "Img already in media library".
That is fine. The modded coded in the background will actually do it's magic.

Once it is finished. You can go to  Media > Library, and you'll see the unattached images are now attached.