al_get_path_extension man page
al_get_path_extension — Allegro 5 API
#include <allegro5/allegro.h> const char *al_get_path_extension(const ALLEGRO_PATH *path)
Return a pointer to the start of the extension of the filename, i.e. everything from the final dot ('.') character onwards. If no dot exists, returns an empty string.
The returned pointer is valid only until the filename part of the path is modified in any way, or until the path is destroyed.
al_get_path_basename(3), al_get_path_filename(3), al_set_path_extension(3).