Accept Unicode str values in parse_options_header

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+* Accept Unicode ``str`` values in ``parse_options_header`` instead of raising ``UnicodeEncodeError`` [#319](https://github.com/Kludex/python-multipart/issues/319).
 * Speed up querystring callback dispatch [#316](https://github.com/Kludex/python-multipart/pull/316).
 
 ## 0.0.32 (2026-06-04)

python_multipart/multipart.py

@@ -193,6 +193,19 @@ def _parseparam(s: str) -> list[str]:
     return plist
 
 
+def _encode_header_token(value: str) -> bytes:
+    """Encode a header token from a Python ``str``.
+
+    HTTP header fields are ISO-8859-1. Values that cannot be encoded that way
+    (for example a Unicode filename passed as ``str``) are encoded as UTF-8 so
+    the public ``str`` API does not raise ``UnicodeEncodeError``.
+    """
+    try:
+        return value.encode("latin-1")
+    except UnicodeEncodeError:
+        return value.encode("utf-8")
+
+
 def parse_options_header(value: str | bytes | None) -> tuple[bytes, dict[bytes, bytes]]:
     """Parses a Content-Type header into a value in the following format: (content_type, {parameters})."""
     if not value:
@@ -207,7 +220,7 @@ def parse_options_header(value: str | bytes | None) -> tuple[bytes, dict[bytes,
 
     # If we have no options, return the string as-is.
     if ";" not in value:
-        return (value.lower().strip().encode("latin-1"), {})
+        return (_encode_header_token(value.lower().strip()), {})
 
     ctype, *segments = _parseparam(value)
     options: dict[bytes, bytes] = {}
@@ -225,8 +238,8 @@ def parse_options_header(value: str | bytes | None) -> tuple[bytes, dict[bytes,
         # just the filename.
         if key == "filename" and (val[1:3] == ":\\" or val[:2] == "\\\\"):
             val = val.split("\\")[-1]
-        options[key.encode("latin-1")] = val.encode("latin-1")
-    return ctype.encode("latin-1"), options
+        options[_encode_header_token(key)] = _encode_header_token(val)
+    return _encode_header_token(ctype), options
 
 
 class Field:

tests/test_multipart.py

@@ -253,6 +253,12 @@ def test_simple(self) -> None:
         self.assertEqual(t, b"application/json")
         self.assertEqual(p, {})
 
+    def test_unicode_filename_str(self) -> None:
+        t, p = parse_options_header('form-data; name="upload"; filename="中文.doc"')
+        self.assertEqual(t, b"form-data")
+        self.assertEqual(p[b"name"], b"upload")
+        self.assertEqual(p[b"filename"], "中文.doc".encode("utf-8"))
+
     def test_blank(self) -> None:
         t, p = parse_options_header("")
         self.assertEqual(t, b"")