Dear Sir,
following code does not seem to work:

// we could open the filesize_t size = f.size();// Allocate a buffer to store contents of the file.std::unique_ptr<char[]> buf(new char[size]);// Read and store file contents in buff.readBytes(buf.get(), size);

I get rubish in buf, i had to rewrite like this to be able to read by SPIFF file:

File f = SPIFFS.open(CONFIG_FILE, "r");
String buffer; /// Create a string buffer to get the file contents
//
Serial.println("Get file");
if (!f) Serial.println("FAIL TO LOAD FILE");
while (f.available()){
buffer = f.readString().c_str();
}
Serial.print("file buffer: ");Serial.println(String(buffer)); // Print buffer which contains file contents

char * doc = new char [buffer.length()+1]; // Define a pointer to a char buffer called doc
std::strcpy (doc, buffer.c_str()); // Copy string buffer to char buffer caled doc

Serial.print("doc: ");Serial.println(String(doc)); // Print doc as string
// Do the joson stuff
StaticJsonDocument<1024> json;
Serial.println("deserialize !");

of course add this in a header file:
// strings and c-strings
#include
#include
#include

Cheers and greetings from the netherland.