node-slack-sdk example showing how to upload a S3 stream

upload-a-s3-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 s3Url = 'https://testbucket.s3-us-west-2.amazonaws.com/some/file/in/S3.mp4';
var s3Stream = // from s3.GetObject call
var fileName = path.basename(url.parse(fileUrl).pathname);

// Slack WebClient uses 'request' module which in turn uses 'form-data' module
// which does seeminly unnecessary check for 'path' property of the stream.
// Related issue: https://github.com/request/request/pull/1964
// Related code:  https://github.com/form-data/form-data/blob/master/lib/form_data.js#L98
// HACK: workaround the problem by setting a bogus path value
s3Stream.path = 'bogus/path/to/workaround/form-data/path/check'

// File upload via file param
var streamOpts = {
  file: s3Stream
};

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