let WebClient = require("@slack/client").WebClient;
let web = new WebClient(accessToken);
let opts = {
    content: fs.readFileSync(imageFilePath),
    filename: 'image.jpg',
    title: 'image.jpg',
    channels: 'demo'
};
web.files.upload(msg.photo.caption, opts, (err, res) => {
    if (err) {
        // no errors here
    } else {
        // here i receive the data about the uploaded file, and there it is listed as plain text
    }
});

// doesn't work (see trace log below)
const image = fs.readFileSync('image.png');
await bot.files.upload('image.png', {
  file: image
});

// text buffer doesn't work too
await bot.files.upload('text.txt', {
  file: new Buffer('hello world');
});

// uploads image as Plain Text (incorrect)
await bot.files.upload('image.png', {
  content: image
});

// works well with streams
const imageStream = fs.createReadStream('image.png')
await bot.files.upload('image.png', {
  file: imageStream
});