Python Flask send_file StringIO blank files : <p>I'm using python 3.5 and flask 0.10.1 and liking it, but having a bit of trouble with send_file. I ultimately want to process a pandas dataframe (from Form data, which is unused in this example but necessary in the future) and send it to download as a csv (without a temp file). The best way to accomplish this I've seen is to us StringIO.</p>

<p>Here is the code I'm attempting to use:</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,\
    attachment_filename = 'a_file.txt', mimetype = 'text/csv')
</code></pre>

<p>A file downloads with the proper name, however the file is completely blank.</p>

<p>Any ideas? Issues with encoding? Has this been answered elsewhere?
Thanks!</p>
