Flutter Web file pick and upload via multipart full example

api_call.dart

//example function body
Map<String, dynamic> fields = {
  "data": filedValue,
};

var request = http.MultipartRequest(
    "POST", Uri.parse('url'),
);

request.files.add(
  http.MultipartFile.fromBytes(
    'file', file.fileBytes,
//  contentType: MediaType('application', 'octet-stream'),
    filename: file.file.name,
  ),
);

fields.forEach((k, v) => request.fields[k] = v);

request.headers.addAll(
  {
    r'Content-Type': 'application/json',
    r'Authorization': Authorization,
  },
);

var streamedResponse = await request.send();
var _result = await http.Response.fromStream(streamedResponse);
var value = yourModelOfResponse.fromJson(jsonDecode(_result.body));
return value;

platform_file_picker.dart

import 'dart:html' as htmlfile;
import 'dart:io';

import 'package:file_picker/file_picker.dart';
import 'package:flutter/foundation.dart';

typedef CallbackForFilePicker = Function(List<dynamic> files);

class PlatformFilePicker {
  startWebFilePicker(CallbackForFilePicker pickerCallback) async {
    if (kIsWeb) {
      htmlfile.InputElement uploadInput = htmlfile.FileUploadInputElement();
      uploadInput.click();

      uploadInput.onChange.listen((e) {
        // read file content as dataURL
        final files = uploadInput.files;
        //was just checking for single file but you can check for multiple one
        if (files.length == 1) {
          final htmlfile.File file = files[0];
          final reader = htmlfile.FileReader();

          reader.onLoadEnd.listen((e) {
            //to upload file we will be needing file bytes as web does not work exactly like path thing
            //and to fetch file name we will be needing file object
            //so created custom class to hold both.
            pickerCallback([FlutterWebFile(file, reader.result)]);
          });
          reader.readAsArrayBuffer(file);
        }
      });
    } else {
      File file = await FilePicker.getFile();
      pickerCallback([file.path]);
    }
  }

  getFileName(dynamic file) {
    if (kIsWeb) {
      return file.file.name;
    } else {
      return file.path.substring(file.lastIndexOf(Platform.pathSeparator) + 1);
    }
  }
}

class FlutterWebFile {
  htmlfile.File file;
  List<int> fileBytes;

  FlutterWebFile(this.file, this.fileBytes);
}