It really doesn't matter.
If you feed .c to a c++ compiler it will compile as cpp, .cc/.cxx is just an alternative to .cpp used by some compilers.
.hpp is an attempt to distinguish header files where there are significant c and c++ differences. A common usage is for the .hpp to have the necessary cpp wrappers or namespace and then include the .h in order to expose a c library to both c and c++.