Python Flask: Send file and variable

I have two servers where one is trying to get a file from the other. I am using Flask get requests to send simple data back and forth (strings, lists, JSON objects, etc.).

I also know how to send just a file, but I need to send an error code with my data.

I'm using something along the following lines:

Server 1:

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

Server 2:

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

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

Server 1:

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

...all of which works fine. However, I want to send an error code variable along with file_data so that Server 1 knows the status (and not the HTTP status, but an internal status code).

Any help is appreciated.