Problem upload an file from sdcard to webserver

[MRcode2102]

Hi my friends
I recorded a wave file with Arduino mega 2560 (microphone is connected to Arduino mega 2560) and now I want to upload the recorded wave file is stored in my sd card to a web server with ESP12-F module which connected to Arduino mega 2560 with serial (UART) port, So I can't remove Arduino mega 2560 because I used of it for recording voices;

Now my problem is in uploading recorded wave file from my sd card (which connected to Arduino mega 2560) to a web server.
The problem is when I sending a file with a less size of 32 kb to the webserver, it's working fine and the file has been uploaded but when the size of my file is bigger than 32 kb, I can't send this file.
I think the problem is from the size of the Serial buffer and I don't know my problem for buffer size or another problem!
additional explanation : ( I used WampServer app as webserver and this library (WifiEspAT.h) for communication with ESP12-F module )

this is Link's from WIFIESPAT.h library (https://github.com/jandrassy/WiFiEspAT) and this is my sketch code
`#include <WiFiEspAT.h>
#include <SD.h>
#include <SPI.h>
//#include <StreamLib.h>

File myFile;

String post_host = "192.168.1.103";
const int post_port = 80;
String url = "/upload_file.php";
char server[] = "192.168.1.103";

// Emulate Serial1 on pins 6/7 if not present
#if defined(ARDUINO_ARCH_AVR) && !defined(HAVE_HWSERIAL1)
//#include <SoftwareSerial.h>
//SoftwareSerial Serial1(6, 7); // RX, TX
#define AT_BAUD_RATE 9600
#else
#define AT_BAUD_RATE 115200
#endif

//const char* server = "arduino.cc";

WiFiClient client;

//format bytes
String formatBytes(unsigned int bytes) {
if (bytes < 1024) {
return String(bytes) + "B";
} else if (bytes < (1024 * 1024)) {
return String(bytes / 1024.0) + "KB";
} else if (bytes < (1024 * 1024 * 1024)) {
return String(bytes / 1024.0 / 1024.0) + "MB";
}
}

void setup() {
Serial.begin(115200);
while (!Serial);

Serial1.begin(AT_BAUD_RATE);
WiFi.init(Serial1);

if (WiFi.status() == WL_NO_MODULE) {
Serial.println();
Serial.println("Communication with WiFi module failed!");
// don't continue
while (true);
}

// waiting for connection to Wifi network set with the SetupWiFiConnection sketch
Serial.println("Waiting for connection to WiFi");
while (WiFi.status() != WL_CONNECTED) {
delay(1000);
Serial.print('.');
}
Serial.println();
Serial.println("Connected to WiFi network.");

//test connect to sd
Serial.print("Initializing SD card...");
if (!SD.begin(53))
{
Serial.println("initialization failed!");
return;
}
Serial.println("initialization done.");

//read file from SD
//define file
myFile = SD.open("data.wav", FILE_READ);

String fileName = myFile.name();
String fileSize = formatBytes(myFile.size());

Serial.println();
Serial.println("file exists");

if (myFile)
{
Serial.println("test file:ok");
// print content length and host
Serial.print("contentLength : ");
Serial.println(fileSize);
Serial.print("connecting to ");
Serial.println(post_host);

// We now create a URI for the request
Serial.println("Connected to server");
Serial.print("Requesting URL: ");
Serial.println(url);

// Make a HTTP request and add HTTP headers
//String boundary = "CustomizBoundarye----";
//change with your content type
String contentType = "audio/x-wav";
String portString = String(post_port);
String hostString = String(post_host);


String requestHead = "--RandomNerdTutorials\r\nContent-Disposition: form-data; name=\"data\"; filename=\"data.wav\"\r\nContent-Type: audio/x-wav\r\n\r\n";

String tail = "\r\n--RandomNerdTutorials--\r\n";


int contentLength =  requestHead.length() + myFile.size() + tail.length();

client.connect(server, 80);

client.println("POST " + url + " HTTP/1.1");
client.println("Host: " + post_host);
client.println("Content-Length: " + String(contentLength, DEC));
client.println("Content-Type: multipart/form-data; boundary=RandomNerdTutorials");
client.println();
client.print(requestHead );

// send myFile:
//this method is for upload data very fast
//and only work in ESP software version after 2.3.0
//if your version is lower than please update
//esp software to last version or use bellow comment code
client.write(myFile);
// create file buffer
const int bufSize = 2048;
byte clientBuf[bufSize];
int clientCount = 0;

while (myFile.available())
{
  clientBuf [clientCount] = myFile.read ();
  clientCount++;
  if (clientCount > (bufSize - 1))
  {
    client.write((const uint8_t *)clientBuf, bufSize);
    clientCount = 0;
  }

}

if (clientCount > 0)
{
  client.write((const uint8_t *)clientBuf, clientCount);

}

// send tail
//      char charBuf3[tail.length() + 1];
//      tail.toCharArray(charBuf3, tail.length() + 1);
client.print(tail);


//Serial.print(charBuf3);

}
else
{
// if the file didn't open, print an error:
Serial.println("error opening test.WAV");
Serial.println("Post Failure");
}

// Read all the lines of the reply from server and print them to Serial

Serial.println("request sent");
String responseHeaders = "";

while (client.connected() ) {
// Serial.println("while client connected");
String line = client.readStringUntil('\n');
Serial.println(line);
responseHeaders += line;
if (line == "\r") {
Serial.println("headers received");
break;
}
}

String line = client.readStringUntil('\n');

Serial.println("reply was:");
Serial.println("==========");
Serial.println(line);
Serial.println("==========");
Serial.println("closing connection");

//close file
myFile.close();

}
//}
void loop()
{}
and this is my php code :<?PHP

$path = "data/";
$path = $path . basename( $_FILES['data']['name']);
if(move_uploaded_file($_FILES['data']['tmp_name'], $path)) {
  echo "The file ".  basename( $_FILES['data']['name']).
  " has been uploaded";
} else{
    echo "There was an error uploading the file, please try again!";

}
?>`

and this is Serial monitor response for wave file bigger and less than 32kb ( screenshot's PNG is attached )

(( and also I tested this code with ESP8266 NODE MCU and I can send a bigger file of about 1MB to the same webserver (WampServer) ))

If possible help me
thanks....