req = requests.post('https://www.otherserver.com/_download_file', data = {'filename':filename})

@app.route('/_download_file', methods = ['POST'])
def download_file():
    filename = requests.form.get('filename')

    file_data = codecs.open(filename, 'rb').read()
    return file_data

with codecs.open(req, 'w') as f:
    f.write('new_file.xyz')