Files with dki file extension can be found as part of DoppleKopf game. Most likely very old and not used nowadays.
DKI file extension - DoppelKopf game data
What is dki file? How to open dki files?
File type specification:
The dki file extension seems to have been in the past used in some game called DoppelKopf.
Probably a card game. No further information about this file type could be found.
This dki file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.