diff --git a/modules/openapi-generator/src/main/resources/python-prior/api_client.mustache b/modules/openapi-generator/src/main/resources/python-prior/api_client.mustache
index f11b14d6d4d..410fa822c60 100644
--- a/modules/openapi-generator/src/main/resources/python-prior/api_client.mustache
+++ b/modules/openapi-generator/src/main/resources/python-prior/api_client.mustache
@@ -298,8 +298,10 @@ class ApiClient(object):
             return obj.isoformat()
         elif isinstance(obj, ModelSimple):
             return cls.sanitize_for_serialization(obj.value)
-        elif isinstance(obj, (list, tuple)):
+        elif isinstance(obj, list):
             return [cls.sanitize_for_serialization(item) for item in obj]
+        elif isinstance(obj, tuple):
+            return tuple(cls.sanitize_for_serialization(item) for item in obj)
         if isinstance(obj, dict):
             return {key: cls.sanitize_for_serialization(val) for key, val in obj.items()}
         raise ApiValueError(
diff --git a/samples/client/petstore/python-prior/petstore_api/api_client.py b/samples/client/petstore/python-prior/petstore_api/api_client.py
index be6a8924910..3c9e7a5f68d 100644
--- a/samples/client/petstore/python-prior/petstore_api/api_client.py
+++ b/samples/client/petstore/python-prior/petstore_api/api_client.py
@@ -286,8 +286,10 @@ class ApiClient(object):
             return obj.isoformat()
         elif isinstance(obj, ModelSimple):
             return cls.sanitize_for_serialization(obj.value)
-        elif isinstance(obj, (list, tuple)):
+        elif isinstance(obj, list):
             return [cls.sanitize_for_serialization(item) for item in obj]
+        elif isinstance(obj, tuple):
+            return tuple(cls.sanitize_for_serialization(item) for item in obj)
         if isinstance(obj, dict):
             return {key: cls.sanitize_for_serialization(val) for key, val in obj.items()}
         raise ApiValueError(
diff --git a/samples/client/petstore/python-prior_disallowAdditionalPropertiesIfNotPresent/petstore_api/api_client.py b/samples/client/petstore/python-prior_disallowAdditionalPropertiesIfNotPresent/petstore_api/api_client.py
index be6a8924910..3c9e7a5f68d 100644
--- a/samples/client/petstore/python-prior_disallowAdditionalPropertiesIfNotPresent/petstore_api/api_client.py
+++ b/samples/client/petstore/python-prior_disallowAdditionalPropertiesIfNotPresent/petstore_api/api_client.py
@@ -286,8 +286,10 @@ class ApiClient(object):
             return obj.isoformat()
         elif isinstance(obj, ModelSimple):
             return cls.sanitize_for_serialization(obj.value)
-        elif isinstance(obj, (list, tuple)):
+        elif isinstance(obj, list):
             return [cls.sanitize_for_serialization(item) for item in obj]
+        elif isinstance(obj, tuple):
+            return tuple(cls.sanitize_for_serialization(item) for item in obj)
         if isinstance(obj, dict):
             return {key: cls.sanitize_for_serialization(val) for key, val in obj.items()}
         raise ApiValueError(
diff --git a/samples/openapi3/client/extensions/x-auth-id-alias/python-prior/x_auth_id_alias/api_client.py b/samples/openapi3/client/extensions/x-auth-id-alias/python-prior/x_auth_id_alias/api_client.py
index e6c0b2e0602..2a0b4462521 100644
--- a/samples/openapi3/client/extensions/x-auth-id-alias/python-prior/x_auth_id_alias/api_client.py
+++ b/samples/openapi3/client/extensions/x-auth-id-alias/python-prior/x_auth_id_alias/api_client.py
@@ -286,8 +286,10 @@ class ApiClient(object):
             return obj.isoformat()
         elif isinstance(obj, ModelSimple):
             return cls.sanitize_for_serialization(obj.value)
-        elif isinstance(obj, (list, tuple)):
+        elif isinstance(obj, list):
             return [cls.sanitize_for_serialization(item) for item in obj]
+        elif isinstance(obj, tuple):
+            return tuple(cls.sanitize_for_serialization(item) for item in obj)
         if isinstance(obj, dict):
             return {key: cls.sanitize_for_serialization(val) for key, val in obj.items()}
         raise ApiValueError(
diff --git a/samples/openapi3/client/petstore/python-prior/petstore_api/api_client.py b/samples/openapi3/client/petstore/python-prior/petstore_api/api_client.py
index c14f1d0f1bc..ad9920c602a 100644
--- a/samples/openapi3/client/petstore/python-prior/petstore_api/api_client.py
+++ b/samples/openapi3/client/petstore/python-prior/petstore_api/api_client.py
@@ -286,8 +286,10 @@ class ApiClient(object):
             return obj.isoformat()
         elif isinstance(obj, ModelSimple):
             return cls.sanitize_for_serialization(obj.value)
-        elif isinstance(obj, (list, tuple)):
+        elif isinstance(obj, list):
             return [cls.sanitize_for_serialization(item) for item in obj]
+        elif isinstance(obj, tuple):
+            return tuple(cls.sanitize_for_serialization(item) for item in obj)
         if isinstance(obj, dict):
             return {key: cls.sanitize_for_serialization(val) for key, val in obj.items()}
         raise ApiValueError(
diff --git a/samples/openapi3/client/petstore/python-prior/tests_manual/test_fake_api.py b/samples/openapi3/client/petstore/python-prior/tests_manual/test_fake_api.py
index 12e391238e6..36a515d49c9 100644
--- a/samples/openapi3/client/petstore/python-prior/tests_manual/test_fake_api.py
+++ b/samples/openapi3/client/petstore/python-prior/tests_manual/test_fake_api.py
@@ -14,8 +14,11 @@ from collections import namedtuple
 import os
 import json
 import unittest
+from pathlib import Path
 from unittest.mock import patch
 
+from urllib3.request import RequestMethods
+
 import petstore_api
 from petstore_api.api.fake_api import FakeApi  # noqa: E501
 from petstore_api.rest import RESTClientObject, RESTResponse
@@ -490,6 +493,48 @@ class TestFakeApi(unittest.TestCase):
         finally:
             file.close()
 
+    def test_upload_with_mime_type(self):
+        """Upload a file, while setting a MIME type for that file.
+
+        Verify that:
+
+        * ``post_params`` may contain a three-tuple in the form ``(file_name, file_handle,
+          file_mime_type)``
+        * This three-tuple is passed to urllib3 as a tuple, without being converted to a list.
+
+        See:
+
+        * https://github.com/OpenAPITools/openapi-generator/issues/14012
+        * https://urllib3.readthedocs.io/en/stable/reference/urllib3.request.html
+        """
+        file_path = Path(__file__, "..", "..", "testfiles", "1px_pic1.png").resolve()
+        file_mime_type = "image/png"
+        with patch.object(RequestMethods, 'request') as request:
+            with open(file_path, mode="rb") as file_handle:
+                request.return_value.status = 200
+                resp = self.api.api_client.call_api(
+                    resource_path="/fake/uploadFile",
+                    method="POST",
+                    header_params={"Content-Type": "multipart/form-data"},
+                    post_params={"file": (file_path.name, file_handle, file_mime_type)},
+                )
+
+        # a single multipart/form-data POST call was made, with a single form field
+        request.assert_called_once()
+        fields = request.call_args.kwargs['fields']
+        self.assertEqual(len(fields), 1, fields)
+        field = fields[0]
+
+        # it is in the form (form_field_name, (filename, filedata, mimetype))
+        self.assertEqual(field[0], "file")
+        self.assertEqual(field[1][0], file_path.name)
+        with open(file_path, mode="rb") as file_handle:
+            self.assertEqual(field[1][1], file_handle.read())
+        self.assertEqual(field[1][2], file_mime_type)
+
+        # the form field value wasn't cast to a list
+        self.assertIsInstance(fields[0][1], tuple)
+
     def test_download_attachment(self):
         """Ensures that file deserialization works"""