Just add the folowing
And Change the SD to SDFS. then it will work
#include "sdfs.h"
server.on("/getdata", HTTP_GET, [](AsyncWebServerRequest *request){
request->send(SDFS, "/subj1_1.txt", "text/plain");
});
A example you can find at the folowing location https://github.com/EmileSpecialProducts/portable-Async-disk-drive