al_set_path_extension - Allegro 5 API

       Replaces  the extension of the path with the given one, i.e. replaces everything from the final dot (`.')
       character onwards, including the dot.  If the filename of the path has no extension,  the  given  one  is
       appended.  Usually the new extension you supply should include a leading dot.

       Returns false if the path contains no filename part, i.e. the filename part is the empty string.

       al_set_path_extension - Allegro 5 API

al_set_path_filename(3alleg5), al_get_path_extension(3alleg5)

Allegro reference manual                                                          al_set_path_extension(3alleg5)

              #include <allegro5/allegro.h>

              bool al_set_path_extension(ALLEGRO_PATH *path, char const *extension)