diff --git a/python-client/wmill/wmill/client.py b/python-client/wmill/wmill/client.py
index 3ac04a0318..c212af21b6 100644
--- a/python-client/wmill/wmill/client.py
+++ b/python-client/wmill/wmill/client.py
@@ -3,6 +3,7 @@ from __future__ import annotations
 import atexit
 import datetime as dt
 import functools
+from io import BufferedReader, BytesIO
 import logging
 import os
 import random
@@ -395,7 +396,7 @@ class Windmill:
                 part_response = self.post(
                     f"/w/{self.workspace}/job_helpers/multipart_download_s3_file",
                     json={
-                        "file_key": s3object.s3,
+                        "file_key": s3object["s3"],
                         "part_number": part_number,
                         "file_size": file_total_size,
                         "s3_resource_path": s3_resource_path,
@@ -413,7 +414,7 @@ class Windmill:
     def write_s3_file(
         self,
         s3object: S3Object | None,
-        file_content: bytes,
+        file_content: BufferedReader | bytes,
         file_expiration: dt.datetime | None,
         s3_resource_path: str | None,
     ) -> S3Object:
@@ -424,26 +425,56 @@ class Windmill:
         from wmill import S3Object
 
         s3_obj = S3Object(s3="/path/to/my_file.txt")
+
+        # for an in memory bytes array:
         file_content = b'Hello Windmill!'
         client.write_s3_file(s3_obj, file_content)
+
+        # for a file:
+        with open("my_file.txt", "rb") as my_file:
+            client.write_s3_file(s3_obj, my_file)
         '''
         """
-        try:
-            result = self.post(
-                f"/w/{self.workspace}/job_helpers/multipart_upload_s3_file",
-                json={
-                    "file_key": s3object.s3 if s3object is not None else None,
-                    "part_content": file_content,
-                    "parts": [],
-                    "is_final": True,
-                    "cancel_upload": False,
-                    "s3_resource_path": s3_resource_path if s3_resource_path != "" else None,
-                    "file_expiration": file_expiration.isoformat() if file_expiration else None,
-                },
-            ).json()
-        except Exception as e:
-            raise Exception("Could not write file to S3") from e
-        return S3Object(s3=result["file_key"])
+        content_reader: BufferedReader | BytesIO
+        if isinstance(file_content, BufferedReader):
+            content_reader = file_content
+        elif isinstance(file_content, bytes):
+            content_reader = BytesIO(file_content)
+        else:
+            raise Exception("Type of file_content not supported")
+
+        file_key = s3object["s3"] if s3object is not None else None
+        parts = []
+        upload_id = None
+        chunk = content_reader.read(5 * 1024 * 1024)
+        if len(chunk) == 0:
+            raise Exception("File content is empty, nothing to upload")
+        while True:
+            chunk_2 = content_reader.read(5 * 1024 * 1024)
+            reader_done = len(chunk_2) == 0
+            try:
+                response = self.post(
+                    f"/w/{self.workspace}/job_helpers/multipart_upload_s3_file",
+                    json={
+                        "file_key": file_key,
+                        "part_content": [b for b in chunk],
+                        "upload_id": upload_id,
+                        "parts": parts,
+                        "is_final": reader_done,
+                        "cancel_upload": False,
+                        "s3_resource_path": s3_resource_path,
+                        "file_expiration": file_expiration.isoformat() if file_expiration else None,
+                    },
+                ).json()
+            except Exception as e:
+                raise Exception("Could not write file to S3") from e
+            parts = response["parts"]
+            upload_id = response["upload_id"]
+            file_key = response["file_key"]
+            if response["is_done"]:
+                break
+            chunk = chunk_2
+        return S3Object(s3=file_key)
 
     def __boto3_connection_settings(self, s3_resource) -> Boto3ConnectionSettings:
         endpoint_url_prefix = "https://" if s3_resource["useSSL"] else "http://"
@@ -706,8 +737,8 @@ def load_s3_file(s3object: S3Object, s3_resource_path: str = "") -> bytes:
 @init_global_client
 def write_s3_file(
     s3object: S3Object | None,
-    file_content: bytes,
-    file_expiration: dt.datetime | None,
+    file_content: BufferedReader | bytes,
+    file_expiration: dt.datetime | None = None,
     s3_resource_path: str = "",
 ) -> S3Object:
     """