.RTFD File Extension

Rich text file similar to an .RTF document, but may contain attachments, such as images, PDFs, fonts, or other files; originally developed for the NeXT operating system and later moved to Mac OS X.

RTFD files are actually "bundles" which contain an .RTF file and the attachments as separate files. In Mac OS X, you can right-click an RTFD file and select "Show Package Contents" to see the files.