node-slack-sdk example showing how to upload a stream

upload-a-stream.js

var path = require('path');
var url = require('url');
var request = require('request');
var WebClient = require('@slack/client').WebClient;

var token = process.env.SLACK_API_TOKEN || '';
var web = new WebClient(token);

var fileUrl = 'http://www.sample-videos.com/video/mp4/720/big_buck_bunny_720p_1mb.mp4';
var fileName = path.basename(url.parse(fileUrl).pathname);

var fileStream = request(fileUrl)
if (!fileStream) throw new Error('could not fetch stream');

// File upload via file param
var streamOpts = {
  file: fileStream
};

web.files.upload(fileName, streamOpts, function handleStreamFileUpload(err, res) {
  if (err) {
    console.error(err);
  } else {
    console.log(res);
  }
});