I can create a .rtfd file with TextEdit by simply adding a graphic to a text file. TextEdit changes the extension to .rtfd. I have no trouble saving and opening these files in a DropBox folder in the Mac Finder. These files are actually packages, folders with contents inside. The .rtfd extension causes the Mac to present them as files as opposed to folders.
Trouble happens when the DropBox folder that contains my .rtfd file is shared with a friend who also uses a Mac. When the friend edits the .rtfd file and moves the .rtfd file to another shared location, the .rtfd file is no longer visible in my Mac Finder. However, I can locate and download the file from DropBox on the web. In other words, the file still exists and its contents are correct, but I cannot see it in the shared folder on my Mac.
When I download the .rtfd file from DropBox on the web, I receive a .tar file. In order to use it, I must decompress it. In the decompression process, the file name is changed to dropbox_download.rtfd.
If I rename the file to its original name and place it in the shared DropBox folder on my Mac, DropBox realizes that there is another (invisible) file in that folder with the same name, and DropBox renames the restored file as a conflicted copy. The conflicted copy no longer has a .rtfd extension and now appears as a folder.