There is only one file type that uses the .UBJ extension:
Universal Binary JSON File
File created in the Universal Binary JSON (UBJ) format, a data interchange format developed by The Buzz Media; stores .JSON data using a binary representation rather than its native text format; allows data to be transferred in a more compact format.
The Universal Binary JSON specification was developed with the following goals in mind: 1) universal compatibility with the JSON specification and with common programming languages, 2) ease of use, and 3) maintaining a small data size while supporting fast parsing capabilities.
Updated: November 18, 2020