fatal error: SPIFFS.h: No such file or directory #include <SPIFFS.h>

The sketch is from

When I compile it for Generic ESP8266 Module, I got the above fatal error message.

The #include <SPIFFS.h> comes from file Configuration.hpp

When I compile it for ESP32 Dev Module, the #include <SPIFFS.h> works fine.

I have done searching for SPIFFS.h (in Windows) in my ..\AppData folder and found, that there is SPIFFS.h in my ...\packages\esp32\hardware...

but in corresponding ...\packages\esp8266\hardware... there is spiffs.h instead.

So I changed the

#include <SPIFFS.h>

to:

#if defined (ESP32)
  #include <SPIFFS.h>
#else  
  #include <spiffs.h>
#endif

but I still get:

fatal error: spiffs.h: No such file or directory