with open('/root/Desktop/fichero.txt', 'rb') as outfile:
    datos = outfile.read()


headers = {'Content-type': 'application/json'}
r = requests.post("http://localhost:5000/api/encrypt", json={'file':base64.encode(datos),'alg':'aes256'}, headers=headers)

print r.content

from Crypto.Cipher import AES

@app.route('/api/encrypt_file', methods=['POST'])
def encrypt_file():

    data = request.json

    data['file'] = base64.b64decode(data['file'])

    iv = Random.new().read(16)

    cipher = AES.new(key,AES.MODE_CBC,iv)#key es byte string de 32 bits
    fichero_enc= iv + cipher.encrypt(data['file'])

    return jsonify({'file':base64.b64encode(fichero_enc)}) #aquí salta la excepción

>> UnicodeDecodeError: 'utf8' codec can't decode byte 0x86 in position 2: invalid start byte