slack.js

const { WebClient } = require('@slack/web-api');
const slackToken = process.env.SLACK_TOKEN;

async function uploadFile() {
  const { createReadStream } = require('fs');
  const web = new WebClient(slackToken);
  const conversationId = '';

  try {
    const result = await web.files.upload({
      channels: conversationId,
      file: createReadStream(`./${fileName}`),
      filename: fileName,
    });
    console.log('File uploaded: ', result.file.id);
  } catch (e) {
    console.error(e);
  }
}

async function postMessage() {
  const web = new WebClient(slackToken);
  const result = await web.text.postMessage({
    text: fileName,
  });
}