This is how i do for download the last firmware of an IoT product to the SD card of my ESP32

ESP32 - Download Firmware from Cloud to SD - "main.cpp"

/**
*
*2018-10-24 Mickael Lehoux
*
*PlatformIO project
*//main.cpp
*
*Used to download firmware to ESP32 SD card from private server
*Use the ESP32Dev web server to start request for the last firmware by using HTTP request
*/

#include "main.h"
#include "FS.h"
#include "SD.h"
#include "HTTPClient.h"
#include "ESP32WebServer.h"

ESP32WebServer webServer(80);

// Functions declarations
void wifiManager(char *ssid, char *pwd);
void getLastFirmware();
void downloadLastFirmware(String uriParam);

// Define CS pin for the SD card module
#define SD_CS 5

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

    // Connect to the WiFi
    wifiManager("you_ssid", "you_password");

// SD card Initialization
// Make SD card always run -> add pull-up on MISO pin
#ifdef ESP32
    //Serial.println(MISO);
    pinMode(19, INPUT_PULLUP);
#endif

    SD.begin(SD_CS);
    if (!SD.begin(SD_CS))
    {
        Serial.println("Card Mount Failed");
        delay(1000);
        ESP.restart();
    }
    uint8_t cardType = SD.cardType();
    if (cardType == CARD_NONE)
    {
        Serial.println("No SD card attached");
        delay(1000);
        ESP.restart();
    }
    Serial.println("Initializing SD card...");
    if (!SD.begin(SD_CS))
    {
        Serial.println("ERROR - SD card initialization failed!");
        delay(1000);
        ESP.restart();
    }

    Serial.println("SD card initialized");

    webServer.on("/getLastFirmware", getLastFirmware);
    webServer.begin();
    Serial.println("HTTP Server started");
}

void loop()
{
    delay(1000);
    Serial.print(".");
    webServer.handleClient();
}

void wifiManager(char *ssid, char *pwd)
{
    WiFi.mode(WIFI_STA);
    WiFi.begin(ssid, pwd);

    while (WiFi.status() != WL_CONNECTED)
    {
        Serial.print(F("."));
        delay(250);
    }
    Serial.println();
    Serial.print(F("Connected to Wifi : "));
    Serial.println(WiFi.SSID());
    Serial.print(F("WiFi IP address: "));
    Serial.println(WiFi.localIP());
}

void getLastFirmware()
{
    Serial.println("getLastFirmware function start");
    // Check if there are some arguments from the GET request
    if (webServer.args() > 0)
    {
        // Find uriParam argument from request
        if (webServer.hasArg("uriParam"))
        {
            downloadLastFirmware(String(webServer.arg(0)));
        }
    }
}

//download last firmware from server and store it on sd card
void downloadLastFirmware(String uriParam)
{
    Serial.println("Enter in downloadLastFirmware function");
    String fwUrlLastVersion = "http://my-server-domain/" + uriParam + "/last/release";

    HTTPClient httpClient;
    httpClient.begin(fwUrlLastVersion);
    int httpCode = httpClient.POST("");
    Serial.println("HTTP reponse code :" + httpCode);

    if (httpCode == HTTP_CODE_OK)
    {
        Serial.print("Current firmware version : ");
        Serial.println(FW_VERSION);

        String newFWVersion = httpClient.getString();

        Serial.print(F("request response from server :"));
        Serial.println(newFWVersion);

        //parsing json response from server
        DynamicJsonBuffer jsonBuffer;
        JsonObject &rootJsonResponse = jsonBuffer.parseObject(newFWVersion);

        if (!rootJsonResponse.success())
        {
            Serial.println(F("parseObject() failed"));
        }
        Serial.println(F("Parsing json response Success"));
        const char *release = rootJsonResponse["release"];
        String newRelease = String(release);

        Serial.print("Available firmware version : ");
        Serial.println(newRelease);

        newRelease.replace(".", "");
        int newVersion = newRelease.toInt();

        String actualFW = FW_VERSION;
        actualFW.replace(".", "");
        int actualVersion = actualFW.toInt();

        if (newVersion > actualVersion)
        {
            WiFiClient fwDlClient;

            String host = "my-server-domain/";
            String url = "/" + uriParam + "/firmware.bin";
            if (fwDlClient.connect(host.c_str(), 80))
            {
                Serial.print("connected to : ");
                Serial.println(host);
                // Make a HTTP request to the server
                fwDlClient.println("GET " + url + " HTTP/1.1"); // change resource to get here
                fwDlClient.println("Host: " + host);            // change resource host here
                fwDlClient.println("Cache-Control: no-cache");
                fwDlClient.println("Connection: close");
                fwDlClient.println();
            }
            else
            {
                Serial.println("Connection to " + host + " failed...");
            }

            //delay the available client test
            delay(250);

            if (fwDlClient.available())
            {
                Serial.println("client is available");
                // Check to see if the file exists
                if (SD.exists("/" + uriParam + "/firmware.bin"))
                {
                    Serial.println("firmware file exists, deleting from sd card");
                    SD.remove("/" + uriParam + "/firmware.bin");
                }
                else
                {
                    Serial.println("firmware file doesn't exist on sd card. Let's create it!");
                }

                File firmwareFile = SD.open("/" + uriParam + "/firmware.bin", FILE_WRITE);
                if (firmwareFile)
                {
                    Serial.println("Writing file on SD card from server...");
                    Serial.println("It's will take 2 or 5min!! be patient please....");
                    size_t currentSize;
                    while (fwDlClient.connected())
                    {
                        byte c = fwDlClient.read();
                        currentSize = firmwareFile.write(c);
                        //Serial.println("...");
                    }
                    // close the file
                    firmwareFile.close();
                    Serial.println("firmware file " + String(currentSize) + " upload done");
                }
                else
                {
                    // if the file didn't open, print an error
                    Serial.println("error opening file");
                }
            }

            if (!fwDlClient.connected())
            {
                Serial.println("disconnecting");
                fwDlClient.flush();
                fwDlClient.stop();
                Serial.println("Finished writing to file");
            }
        }
        else
        {
            Serial.println("Already on latest version");
        }
    }
    else
    {
        Serial.print("Firmware version check failed, got HTTP response code : ");
        Serial.println(httpCode);
    }
    httpClient.end();
}

ESP32 - Download Firmware from Cloud to SD - "main.h"

#include "Arduino.h"
#include "ArduinoJson.h"
#include "WiFi.h"

#define FW_VERSION "0.0.1"

extern String newRelease;