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File upload handler

Uploading file from Web Browser

If you have to write some data individually to the ESP8266/ESP32 module for the Sketch behavior, the AutoConnectFile element will assist with your wants implementation. The AutoConnectFile element produces an HTML <input type="file"> tag and can save uploaded file to the flash or external SD of the ESP8266/ESP32 module. The handler for saving is built into AutoConnect. You can use it to inject any sketch data such as the initial values for the custom Web page into the ESP module via OTA without using the Sketch data upload tool of Arduino-IDE.

Basic steps of the file upload sketch

Here is the basic procedure of the Sketch which can upload files from the client browser using AutoConnectFile:1

  1. Place AutoConnectFile on a custom Web page by writing JSON or constructor code directly with the Sketch.
  2. Place other AutoConnectElements as needed.
  3. Place AutoConnectSubmit on the same custom Web page.
  4. Perform the following process in the on-handler of submitting destination:
    • Retrieve the AutoConnectFile instance from the custom Web page where you placed the AutoConnectFile element using the AutoConnectAux::getElement function or the operator [].
    • Start access to the device specified as the upload destination. In usually, it depends on the file system's begin function. For example, if you specified Flash's SPIFFS as the upload destination, invokes SPIFFS.begin().
    • The value member of AutoConnectFile contains the file name of the upload file. Use its file name to access the uploaded file on the device.
    • Invokes the end function associated with the begin to close the device. It is the SPIFFS.end()* if the flash on the ESP module has been begun for SPIFFS.

The following sketch is an example that implements the above basic steps. The postUpload function is the on-handler and retrieves the AutoConnectFile as named upload_file. You should note that this handler is not for a custom Web page placed with its AutoConnectFile element. The uploaded file should be processed by the handler for the transition destination page from the AutoConnectFile element placed page. AutoConnect built-in upload handler will save the uploaded file to the specified device before invoking the postUpload function.

However, If you use uploaded files in different situations, it may be more appropriate to place the actual handling process outside the handler. It applies for the parameter file, etc. The important thing is that you do not have to sketch file reception and storing logic by using the AutoConnectFile element and the upload handler built into the AutoConnect.

#include <ESP8266WiFi.h>
#include <ESP8266WebServer.h>
#include <FS.h>
#include <AutoConnect.h>

// Upload request custom Web page
static const char PAGE_UPLOAD[] PROGMEM = R"(
  "uri": "/",
  "title": "Upload",
  "menu": true,
  "element": [
    { "name":"caption", "type":"ACText", "value":"<h2>File uploading platform<h2>" },
    { "name":"upload_file", "type":"ACFile", "label":"Select file: ", "store":"fs" },
    { "name":"upload", "type":"ACSubmit", "value":"UPLOAD", "uri":"/upload" }

// Upload result display
static const char PAGE_BROWSE[] PROGMEM = R"(
  "uri": "/upload",
  "title": "Upload",
  "menu": false,
  "element": [
    { "name":"caption", "type":"ACText", "value":"<h2>Uploading ended<h2>" },
    { "name":"filename", "type":"ACText" },
    { "name":"size", "type":"ACText", "format":"%s bytes uploaded" },
    { "name":"content_type", "type":"ACText", "format":"Content: %s" }

ESP8266WebServer server;
AutoConnect portal(server);
// Declare AutoConnectAux separately as a custom web page to access
// easily for each page in the post-upload handler.
AutoConnectAux auxUpload;
AutoConnectAux auxBrowse;

 * Post uploading, AutoConnectFile's built-in upload handler reads the
 * file saved in SPIFFS and displays the file contents on /upload custom
 * web page. However, only files with mime type uploaded as text are
 * displayed. A custom web page handler is called after upload.
 * @param  aux  AutoConnectAux(/upload)
 * @param  args PageArgument
 * @return Uploaded text content
String postUpload(AutoConnectAux& aux, PageArgument& args) {
  String  content;
  AutoConnectFile&  upload = auxUpload["upload_file"].as<AutoConnectFile>();
  AutoConnectText&  aux_filename = aux["filename"].as<AutoConnectText>();
  AutoConnectText&  aux_size = aux["size"].as<AutoConnectText>();
  AutoConnectText&  aux_contentType = aux["content_type"].as<AutoConnectText>();
  // Assignment operator can be used for the element attribute.
  aux_filename.value = upload.value;
  aux_size.value = String(upload.size);
  aux_contentType.value = upload.mimeType;
  // The file saved by the AutoConnect upload handler is read from
  // the EEPROM and echoed to a custom web page.
  File uploadFile ="/" + upload.value).c_str(), "r");
  if (uploadFile) {
    while (uploadFile.available()) {
      char c =;
    content = "Not saved";
  return String();

void setup() {

  portal.join({ auxUpload, auxBrowse });

void loop() {

Where will the file upload

The AutoConnect built-in upload handler can save the upload file to three locations:

  1. Flash memory embedded in the ESP8266/ESP32 module
  2. SD device externally connected to the ESP8266/ESP32 module
  3. Other character devices

You can specify the device type to save with the store attribute of AutoConnectFile, and it accepts the following values:

  • Flash : AC_File_FS for the API parameter or fs for the JSON document
  • SD : AC_File_SD for the API parameter or sd for the JSON document
  • Other : AC_File_Extern for the API parameter or extern for the JSON document

The substance of AC_File_FS (fs) is a SPIFFS file system implemented by the ESP8266/ESP32 core, and then AutoConnect uses the Global Instance SPIFFS to access SPIFFS.

Also, the substance of AC_File_SD (sd) is a FAT file of Arduino SD library ported to the ESP8266/ESP32 core, and then AutoConnect uses the Global Instance SD to access SD. When saving to an external SD device, there are additional required parameters for the connection interface and is defined as the macro in AutoConnectDefs.h.

#define AUTOCONNECT_SD_CS       SS
#define AUTOCONNECT_SD_SPEED    4000000

AUTOCONNECT_SD_CS defines which GPIO for the CS (Chip Select, or SS as Slave Select) pin. This definition is derived from pins_arduino.h, which is included in the Arduino core distribution. If you want to assign the CS pin to another GPIO, you need to change the macro definition of AutoConnectDefs.h.

AUTOCONNECT_SD_SPEED defines SPI clock speed depending on the connected device.

Involves both the begin() and the end()

The built-in uploader executes the begin and end functions regardless of the Sketch whence the file system of the device will terminate with the uploader termination. Therefore, to use the device in the Sketch after uploading, you need to restart it with the begin function.

When it will be uploaded

Upload handler will be launched by ESP8266WebServer/WebServer(as ESP32) library which is triggered by receiving an HTTP stream of POST BODY including file content. Its launching occurs before invoking the page handler.

The following diagram illustrates the file uploading sequence:

At the time of the page handler behaves, the uploaded file already saved to the device, and the member variables of AutoConnectFile reflects the file name and transfer size.

The file name for the uploaded file

AutoConnetFile saves the uploaded file with the file name you selected by <input type="file"> tag on the browser. The file name used for uploading is stored in the AutoConnetFile's value member, which you can access after uploading. (i.e. In the handler of the destination page by the AutoConnectSubmit element.) You can not save it with a different name. It can be renamed after upload if you need to change the name.

Upload to a device other than Flash or SD

You can output the file to any device using a custom uploader by specifying extern with the store attribute of AutoConnectFile (or specifying AC_File_Extern for the store member variable) and can customize the uploader according to the need to upload files to other than Flash or SD. Implements your own uploader with inheriting the AutoConnectUploadHandler class which is the base class of the upload handler.

It's not so difficult

Implementing the custom uploader requires a little knowledge of the c++ language. If you are less attuned to programming c++, you may find it difficult. But don't worry. You can make it in various situations by just modifying the Sketch skeleton that appears at the end of this page.

Upload handler base class

AutoConnectUploadHandler is a base class of upload handler and It has one public member function and three protected functions.



Member functions

The upload public function is an entry point, the ESP8266WebServer (WebServer as ESP32) library will invoke the upload with each time of uploading content divided into chunks.

Also, the _open, _write and _close protected functions are actually responsible for saving files and are declared as pure virtual functions. A custom uploader class that inherits from the AutoConnectUploadHandler class need to implement these functions.

The actual upload process is handled by the three private functions above, and then upload only invokes three functions according to the upload situation. In usually, there is no need to override the upload function in an inherited class.

public virtual void upload(const String& requestUri, const HTTPUpload& upload)
requestUriURI of upload request source.
uploadA data structure of the upload file as HTTPUpload. It is defined in the ESP8266WebServer (WebServer as ESP32) library as follows:
typedef struct {
  HTTPUploadStatus status;
  String  filename;
  String  name;
  String  type;
  size_t  totalSize;
  size_t  currentSize;
  size_t  contentLength;
  uint8_t buf[HTTP_UPLOAD_BUFLEN];
} HTTPUpload;

An upload handler needs to implement a procedure corresponding with HTTPUploadStatus enum value indicated by the uploading process of ESP8266WebServer class, which contained in HTTPUpload.status as following values:

  • UPLOAD_FILE_START : Invokes to the _open.
  • UPLOAD_FILE_WRITE : Invokes to the _write.
  • UPLOAD_FILE_END : Invokes to the _close.
  • UPLOAD_FILE_ABORTED : Invokes to the _close.

The _open function will be invoked when HTTPUploadStatus is UPLOAD_FILE_START. Usually, the implementation of an inherited class will open the file.

protected virtual bool _open(const char* filename, const char* mode) = 0
filenameUploading file name.
modeAn indicator for the file access mode, a "w" for writing.
Return value
trueFile open successful.
falseFailed to open.

The _write function will be invoked when HTTPUploadStatus is UPLOAD_FILE_WRITE. The content of the upload file is divided and the _write will be invoked in multiple times. Usually, the implementation of an inherited class will write data.

protected virtual size_t _write(const uint8_t *buf, const size_t size) = 0
bufFile content block.
sizeFile block size to write.
Return value
Size written.

The _close function will be invoked when HTTPUploadStatus is UPLOAD_FILE_END or UPLOAD_FILE_ABORTED. Usually, the implementation of an inherited class will close the file.

protected virtual void _close(void) = 0

For reference, the following AutoConnectUploadFS class is an implementation of AutoConnect built-in uploader and inherits from AutoConnectUploadHandler.

class AutoConnectUploadFS : public AutoConnectUploadHandler {
  explicit AutoConnectUploadFS(SPIFFST& media) : _media(&media) {}
  ~AutoConnectUploadFS() { _close(); }

  bool _open(const char* filename, const char* mode) override {
    if (_media->begin()) {
      _file = _media->open(filename, mode);
      return _file != false;      
    return false;

  size_t _write(const uint8_t* buf, const size_t size) override {
    if (_file)
      return _file.write(buf, size);
      return -1;

  void _close(void) override {
    if (_file)

  SPIFFST*  _media;
  SPIFileT  _file; 

Register custom upload handler

In order to upload a file by the custom uploader, it is necessary to register it to the custom Web page beforehand. To register a custom uploader, specify the custom uploader class name in the template argument of the AutoConnectAux::onUpload function and invokes it.

void AutoConnectAux::onUpload<T>(T& uploadClass)
TSpecifies a class name of the custom uploader. This class name is a class that you implemented by inheriting AutoConnectUploadHandler for custom upload.
uploadClassSpecifies the custom upload class instance.

The rough structure of the Sketches that completed these implementations will be as follows:

#include <ESP8266WiFi.h>
#include <ESP8266WebServer.h>
#include <AutoConnect.h>

static const char PAGE_UPLOAD[] PROGMEM = R"(
  "uri": "/",
  "title": "Upload",
  "menu": true,
  "element": [
    { "name":"caption", "type":"ACText", "value":"<h2>File uploading platform<h2>" },
    { "name":"upload_file", "type":"ACFile", "label":"Select file: ", "store":"extern" },
    { "name":"upload", "type":"ACSubmit", "value":"UPLOAD", "uri":"/upload" }

static const char PAGE_RECEIVED[] PROGMEM = R"(
  "uri": "/upload",
  "title": "Upload ended",
  "menu": false,
  "element": [
    { "name":"caption", "type":"ACText", "value":"<h2>File uploading ended<h2>" }

// Custom upload handler class
class CustomUploader : public AutoConnectUploadHandler {
  CustomUploader() {}
  ~CustomUploader() {}

  bool   _open(const char* filename, const char* mode) override;
  size_t _write(const uint8_t *buf, const size_t size) override;
  void   _close(void) override;

// _open for custom open
bool CustomUploader::_open(const char* filename, const char* mode) {
  // Here, an implementation for the open file.

// _open for custom write
size_t CustomUploader::_write(const uint8_t *buf, const size_t size) {
  // Here, an implementation for the writing the file data.

// _open for custom close
void CustomUploader::_close(void) {
  // Here, an implementation for the close file.

AutoConnect     portal;
AutoConnectAux  uploadPage;
AutoConnectAux  receivePage;
CustomUploader  uploader;   // Declare the custom uploader

void setup() {
  portal.join({ uploadPage, receivePage });
  receivePage.onUpload<CustomUploader>(uploader);  // Register the custom uploader

void loop() {

Don't forget to specify the store

When using a custom uploader, remember to specify the extern for the store attribute of AutoConnectFile.

  1. The AutoConnectFile element can be used with other AutoConnectElements on the same page.