At the end of the day it doesn't matter because C++ compilers can deal with the files in either format. If it's a real issue within your team, flip a coin and move on to the actual work.
GNU GCC recognises all of the following as C++ files, and will use C++ compilation regardless of whether you invoke it through gcc or g++:
.C - case matters in GCC,
.c is a C file whereas
.C is a C++ file (if you let the compiler decide what it is compiling that is).
GCC also supports other suffixes to indicate special handling, for example a
.ii file will be compiled as C++, but not pre-processed (intended for separately pre-processed code). All the recognised suffixes are detailed at gcc.gnu.org