The server doesn't complete my file uploads that are larger than 2MB

[Shimoe-Koharu16]

Platform

ESP32

IDE / Tooling

Arduino (IDE/CLI)

What happened?

Whenever I try to upload a file that's larger than 2MB, it doesn't continue and leaves an incomplete file data (corrupt file in other words). To be specific, I was trying to upload a file that's 28MB+ on size (ZIP file), but when I'm about to extract the file on my PC or phone (after downloading the file/ZIP folder), it doesn't open with any file explorers I use (Nautilus and Files). The Serial output says that the upload was a success but it doesn't actually finished writing on the SD card. I already added .flush() to rule out RAM limitations (but the average RAM use while upload is ~210kB based on the dashboard on Blynk that I set up on my ESP32 as an RTOS task). But, it still does produce the same bug or problem

Stack Trace

------------------------------------------
  Compile Date/Time : Oct  7 2025 18:40:59
  Compile Host OS   : linux
  ESP-IDF Version   : v5.5.1-1-g129cd0d247
  Arduino Version   : 3.3.0
------------------------------------------
Board Info:
------------------------------------------
  Arduino Board     : ESP32_DEV
  Arduino Variant   : esp32
  Arduino FQBN      : esp32:esp32:esp32:UploadSpeed=921600,CPUFreq=240,FlashFreq=80,FlashMode=qio,FlashSize=4M,PartitionScheme=default,DebugLevel=debug,PSRAM=disabled,LoopCore=1,EventsCore=1,EraseFlash=none,JTAGAdapter=default,ZigbeeMode=default
============ Before Setup End ============
Server Initialized
=========== After Setup Start ============
[  7369][D][NetworkManager.cpp:83] hostByName(): Clearing DNS cache
INTERNAL Memory Info:
------------------------------------------
  Total Size        :   317784 B ( 310.3 KB)
  Free Bytes        :   155816 B ( 152.2 KB)
  Allocated Bytes   :   150304 B ( 146.8 KB)
  Minimum Free Bytes:   155772 B ( 152.1 KB)
  Largest Free Block:    94196 B (  92.0 KB)
------------------------------------------
GPIO Info:
------------------------------------------
  GPIO : BUS_TYPE[bus/unit][chan]
  --------------------------------------  
     1 : UART_TX[0]
     2 : GPIO
     3 : UART_RX[0]
     5 : SD_SS
    17 : GPIO
    18 : SPI_MASTER_SCK[3]
    19 : SPI_MASTER_MISO[3]
    23 : SPI_MASTER_MOSI[3]
============ After Setup End =============
[  7486][D][NetworkManager.cpp:127] hostByName(): DNS found IPv4 128.199.144.129
Upload Start: てるくず.zip
Upload Success: てるくず.zip

Minimal Reproductible Example (MRE)

Here's the upload endpoint for my website to send the files to
  server.on(
    "/upload", HTTP_POST, [](AsyncWebServerRequest *request) {
      request->send(204);
    },
    handleUpload);

And here's my upload handler 

`void handleUpload(AsyncWebServerRequest *request, const String &filename,
                  size_t index, uint8_t *data, size_t len, bool final) {

  if (index == 0) {
    if (!enable_upload) {
      events.send("Upload disabled from the the dashboard", "toast", millis());
      return;
    }
    Serial.printf("Upload Start: %s\n", filename.c_str());
    fn = filename;
    String path = "/uploads/" + filename;
    uploadFile = SD.open(path, FILE_WRITE);
    if (!uploadFile) {
      Serial.println("Failed to open file for writing");
      return;
    }
  }
  if (!enable_upload) {
    events.send("Upload disabled from the the dashboard", "toast", millis());
    return;
  }
  if (uploadFile) {
    if (len > 0)
      uploadFile.write(data, len);
      uploadFile.flush();
      digitalWrite(2, HIGH);
    if (final) {
      uploadFile.close();
      Serial.printf("Upload Success: %s\n", filename.c_str());
    }
    digitalWrite(2, LOW);
  }
}`

I confirm that:

• I have read the documentation.
• I have searched for similar discussions.
• I have searched for similar issues.
• I have looked at the examples.
• I have upgraded to the lasted version of ESPAsyncWebServer (and AsyncTCP for ESP32).