void setup() {
  // put your setup code here, to run once:
  String stringOne = "Content-Type: text/html";
  Serial.println(stringOne);

  // substring(index) looks for the substring from the index position to the end:
  if (stringOne.substring(19) == "html") {
    Serial.println("It's an html file");
  }
  // you can also look for a substring in the middle of a string:
  if (stringOne.substring(14, 18) == "text") {
    Serial.println("It's a text-based file");
  }
}

void loop() {
  // put your main code here, to run repeatedly:
  delay(10); // this speeds up the simulation
}