Rich Text Format Directory or Rich Text Format with Attachments is a primary document format of TextEdit, an application native to NeXTSTEP and Mac OS X which has also been ported to other versions of Unix. The file format is based on the Rich Text Format, but can also include "attachments" such as images.
An RTFD file is a bundle, containing multiple files. It contains a Rich Text file called TXT.rtf that contains Rich Text formatting commands, as well as commands for including images or other attachments contained within the bundle. Images used in the document are stored in the bundle in their native formats.