title: Python Flask send_file StringIO blank files
body: <p>I want to process a Pandas dataframe and send it to download as a CSV without a temp file. The best way to accomplish this I've seen is to use <code>StringIO</code>. Using the code below, a file downloads with the proper name, however the file is completely blank, and no error is shown. Why doesn't the file contain data?</p>
  <pre><code>@app.route('/test_download', methods = ['POST'])
  def test_download():
      buffer = StringIO()
      buffer.write('Just some letters.')
      buffer.seek(0)
      return send_file(
          buffer,
          as_attachment=True,
          download_name='a_file.txt',
          mimetype='text/csv'
      )
  </code></pre>