Files with big5 extension are used by the Chinese BIG5 format. They include encoded text data.
BIG5 file extension - Chinese text
What is big5 file? How to open big5 files?
File type specification:
The big5 file extension is associated with the Chinese BIG5 (Big-5) format used to encode traditional Chinese characters to text files. It is used in Taiwan, Hong Kong, and Macau. The .big5 file contains text encoded in BIG-5 format.