title: no_file_data error when using Slack API upload
body: <p>I'm getting this error {"ok":false,"error":"no_file_data"} when I try to call the slack API to upload. I already have the specified file in ./log. </p>
  
  <pre><code>robot.respond /upload (.*)/i, (res) -&gt;
      app_name = res.match[1]
      request = require("request")
      fs = require("fs")
      channel_name = res.message.room
  
      data = {
          channels: channel_name,
          filename: "#{app_name}.txt",
          file: fs.createReadStream("./log/#{app_name}.txt"),
      }
  
      robot.http("https://slack.com/api/files.upload")
        .headers(Authorization: 'Bearer slack-token', 'Content-Type': 'application/json')
        .post(data) (err, httpRes, body) -&gt;
          if err
            res.send "#{err}"
            return
  
          res.send "#{body}"
  </code></pre>