Files with kostya suffix can be typically encountered as encrypted archives that had their extension renamed by Kostya ransomware.
KOSTYA file extension - Kostya ransomware affected file
What is kostya file? How to open kostya files?
File type specification:
File extension kostya seems to be related to a special ransomware that targets predominantly Czech users.
Much like similar ransomware it encrypts users documents and other personal files and demands ransom (2000 CZK) for the encryption key.