Build warns about QML code in C++ header with extension ".hh"

We have our headers in files with extension ".hh". Now when we build, there is a warning:

 
Class SomeClass is declared in SomeClass.hh, which appears not to be a header. The compilation of its registration to QML may fail.
 
I could trace this to the file qtdeclarative/src/qmltyperegistrar/metatypesjsonprocessor.cpp, but it seems to be hard coded for certain extensions. Are there really problems with using other file extensions? I tried to find something about it in the documentation, but could not find anything (searched with google for "QML valid header file extension" and related searches).