diff --git a/api-openbis-python3-pybis/src/python/pybis/dataset.py b/api-openbis-python3-pybis/src/python/pybis/dataset.py
index 6018f69c3aa8a82336add829c6150b999dbc7bea..1c20fb0a6ad21129f96b07dcb9de70d8fd307bb7 100644
--- a/api-openbis-python3-pybis/src/python/pybis/dataset.py
+++ b/api-openbis-python3-pybis/src/python/pybis/dataset.py
@@ -29,7 +29,7 @@ from urllib.parse import urljoin, quote
 import requests
 from pandas import DataFrame
 from requests import Session
-# from requests_toolbelt.multipart.encoder import MultipartEncoder
+from requests_toolbelt.multipart.encoder import MultipartEncoder
 from tabulate import tabulate
 
 from .definitions import (
@@ -1246,16 +1246,16 @@ class DataSetUploadQueue:
             file_size = os.path.getsize(filename)
 
             if self.multipart is True:
-                file = {filename: open(filename, "rb")}
-                resp = requests.post(upload_url, files=file, verify=verify_certificates)
-                resp.raise_for_status()
-                # with open(filename, "rb") as f:
-                #     m = MultipartEncoder(
-                #         fields={filename: (filename, f, 'application/octet-stream')})
-                #     headers = {'Content-Type': m.content_type}
-                #     r = requests.post(upload_url, data=m, headers=headers,
-                #                       verify=verify_certificates)
-                #     r.raise_for_status()
+                # file = {filename: open(filename, "rb")}
+                # resp = requests.post(upload_url, files=file, verify=verify_certificates)
+                # resp.raise_for_status()
+                with open(filename, "rb") as f:
+                    m = MultipartEncoder(
+                        fields={filename: (filename, f, 'application/octet-stream')})
+                    headers = {'Content-Type': m.content_type}
+                    r = requests.post(upload_url, data=m, headers=headers,
+                                      verify=verify_certificates)
+                    r.raise_for_status()
             else:
                 # upload the file to our DSS session workspace
                 with open(filename, "rb") as f: