ESP8266 Server File up- and download über UART

#include <ESP8266WiFi.h>
#include <ESP8266WebServer.h>
#include <WiFiClient.h>
#include <ESP8266WiFiMulti.h>
#include <WiFiUdp.h>
#include <ArduinoOTA.h>
#include <ESP8266mDNS.h>
#include <FS.h>
#include <EEPROM.h>
#include "mainpage.h" //Our HTML webpage contents


//On board LED Connected to GPIO2
#define LED 2 



const char *ssid = "AventuraAero";
char *password = "123456789";
 
ESP8266WebServer server(80);

//--------------Handle Routins --------------
void handleRoot() {
 Serial.println("You called root page");
 String s = MAIN_page; //Read HTML contents
 server.send(200, "text/html", s); //Send web page
}

void handleDownload(){
   //first create a fixed buffer
  const int bufferSize = 6000;
  const uint8_t _buffer[6000];
  
  //a little counter to know at which position we are //in our buffer
  int bufferCounter = 0;
    
  int fileSize = 80000;

  //now send an empty string but with the header field //Content-Length to let the browser know how much data //it should expect
  server.sendHeader("Content-Length", (String)fileSize);
  server.send(200, "text/html", "");
  
  for(int i=0;i<fileSize;i++){
    _buffer[bufferCounter] = random(256); //write some random bytes into our buffer
    bufferCounter++;
    
    if(bufferCounter >= bufferSize){ //when the buffer is full...
      server.sendContent_P(_buffer, bufferCounter); //send the current buffer
      bufferCounter = 0; //reset the counter
    }
    
  }
  

}

void handleUpdate() { 
 Serial.println("Update page");
 digitalWrite(LED,HIGH); //LED off
 server.send(200, "text/html", "Update"); //Send ADC //value only to client ajax request
}

void handleChange_Password() { 
 Serial.println("Change_Password page");
 digitalWrite(LED,HIGH); //LED off
 server.send(200, "text/html", "Change"); //Send ADC //value only to client ajax request
}

//--------------Setup --------------
void setup(void){
  
  Serial.begin(115200);
  EEPROM.begin(64);
  SPIFFS.begin();
  Serial.println("");
  
  //Onboard LED port Direction output
  pinMode(LED,OUTPUT); 
  //Power on LED state off
  digitalWrite(LED,HIGH);
  
  WiFi.mode(WIFI_AP);           //Only Access point
  WiFi.softAP(ssid, password);  //Start HOTspot //removing password will disable security
 
  IPAddress myIP = WiFi.softAPIP(); //Get IP address
  Serial.print("HotSpt IP:");
  Serial.println(myIP);
 
  server.on("/", handleRoot);      //Which routine to //handle at root location. This is display page
  
  server.on("/Download", handleDownload);
  server.on("/Update", handleUpdate);
  server.on("/Change_Password", handleChange_Password);
  
 
  server.begin();                  //Start server
  
  Serial.println("HTTP server started");
}
//--------------Loop --------------
void loop() {
  server.handleClient();
}