Some of the files that I downloaded, mostly those from the CIV3 official web site, have the file extension *.BIX rather than the *.BIC file that CIV3 accepts. Does anyone know how to fix these?
I have Civ3 regular (not PTW) version 1.29f.

I have a sample BIX file in the attachment.

.bix files are the .bic files for PTW. Without PTW, you cannot use them.