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.

Gabriel Lee

WP Attach allows you to attach files to posts or pages as an information box by using the shortcode [wpattach link=”LINK_TO_YOUR_FILE”]. Filetypes supported includes: .jpg .png .mp3 .mp4 .pdf .ae .ai .avi .css .doc .docx .docs .dw .eps .fl .fla .fw .html .id .ps .psd .ppt .pptx .xls .xlsx .slides .sheets

WP Attach

WP Attach allows you to attach files to posts or pages as an information box by using simple shortcodes.

CIT Media Sync vs WP Attach