Use `_TYPE_FIELD_VALUE`  in `urllib3.fields.format_header_param_rfc2231` for consistency
@@ -34,7 +34,7 @@ def guess_content_type(
return default
-def format_header_param_rfc2231(name: str, value: Union[str, bytes]) -> str:
+def format_header_param_rfc2231(name: str, value: _TYPE_FIELD_VALUE) -> str:
"""
Helper function to format and quote a single header parameter using the
strategy defined in RFC 2231.
