Ooypunk

Once upon a time there was a website in the auld style, with imagery in an images directory. But then it was said that 
the website should be built anew, but in WordPress, and the images were transported by FTP to the new structure. But 
the new pages and posts needed images, but the images were not in the Library. 
This plugin scans the upload directory for images that are not yet in the Library, and provides links for each one to 
be included.

CIT Media Sync

Plugin to add files to the Media Library that were previously uploaded by FTP.

This plugin will allow you to attach a single file to multiple posts, but will also will allow you to detach any file.
<ul>
<li>Support http://xparkmedia.com/plugins/file-un-attach/</li>
<li>Languages http://xparkmedia.com/plugins/file-un-attach/#languages</li>
</ul>

File Un-Attach

This plugin will allow you to attach a single file to multiple posts, but will also will allow you to detach any file.

CIT Media Sync vs File Un-Attach