diff --git a/python_multipart/multipart.py b/python_multipart/multipart.py
index 5237786..2a51d10 100644
--- a/python_multipart/multipart.py
+++ b/python_multipart/multipart.py
@@ -539,7 +539,7 @@ def _get_disk_file(self) -> BufferedRandom:
             # TODO: what happens if we don't have a filename?
             fname = self._file_base + self._ext if keep_extensions else self._file_base
 
-            path = os.path.join(file_dir, fname)  # type: ignore[arg-type]
+            path = os.path.join(os.fsencode(file_dir), fname)
             try:
                 self.logger.info("Opening file: %r", path)
                 tmp_file = open(path, "w+b")
diff --git a/tests/test_file.py b/tests/test_file.py
index a2aa134..9fe570f 100644
--- a/tests/test_file.py
+++ b/tests/test_file.py
@@ -24,3 +24,21 @@ def test_upload_dir_with_leading_slash_in_filename(tmp_path: Path) -> None:
     assert not file.in_memory
     assert Path(upload_dir / "foo.txt").exists()
     assert Path(upload_dir / "foo.txt").read_bytes() == b"123456789012"
+
+
+def test_str_upload_dir_with_keep_filename(tmp_path: Path) -> None:
+    file = File(
+        b"foo.txt",
+        config={
+            "UPLOAD_DIR": str(tmp_path),
+            "UPLOAD_KEEP_FILENAME": True,
+            "UPLOAD_KEEP_EXTENSIONS": True,
+            "MAX_MEMORY_FILE_SIZE": 10,
+        },
+    )
+    file.write(b"123456789012")
+    file.close()
+
+    assert not file.in_memory
+    assert file.actual_file_name == b"foo.txt"
+    assert (tmp_path / "foo.txt").read_bytes() == b"123456789012"