title: Slack API files.upload without preview (or non-plaintext)
body: <p>I've been fighting slack's files.upload API. Even if I insist the file should be any kind of format that isn't plain text, example:</p>
  
  <pre><code>    request.post({
        url: 'https://slack.com/api/files.upload',
        formData: {
          token: bot.config.token,
          channels: message.channel,
          file: fs.createReadStream(realLocalPath),
          filetype: 'zip',
          mode: 'hosted'
        },
      }, function (err, response) {
          console.log(JSON.parse(response.body));
      });
  </code></pre>
  
  <p>It seems to insist on making it plaintext with preview:</p>
  
  <pre><code>     mimetype: 'text/plain',
       filetype: 'text',
       pretty_type: 'Plain Text',
       ...
       mode: 'snippet'
  </code></pre>
  
  <p>That's from the API response's file property. Does anyone know how I can override this?</p>
  
  <p>I want my file to be a downloadable one. No snippet, no preview, just a downloadable file.</p>