'String' has not been declared error

Hello,

I have made a simple script and i have just include a cpp file where i declare a String variable.

I have another error but it is still linked to the string library:

"test.cpp:1: error: 'String' does not name a type

String teststring = "ABCDE";"

Main

#include "test.cpp"

void setup() {
  // Open serial communications and wait for port to open:
  Serial.begin(115200);


  // send an intro:
  Serial.println("\n\nString  substring():");
  Serial.println();
}

void loop() {
  // Set up a String:
  String stringOne = "text/html";
  Serial.println(stringOne);


}

test.cpp

String teststring = "ABCDE";