c++ file with no extension

Question

I have a project that throw compilation errors, I have a file named "UmfPackSupport" that is included in another file named "PoissonBlender.hpp" with the line:

#include "unsupported/Eigen/UmfPackSupport"

I then have an error in the line:

solve<double, UmfPackSupport>(A, b, u);

that says

'UmfPackSupport' was not declared in this scope

what is that file with no extension ??