Unhandled LookupError in multipart parser when RFC 2231 encoding name is invalid

[sammiee5311]

When a multipart form upload includes an RFC 2231 encoded filename* parameter with an invalid encoding name (e.g., filename*=BOGUS''test%20file.txt), ​parse_header_parameters(), ​django/utils/http.py passes the encoding to urllib.parse.unquote(), which raises LookupError.

The caller in ​django/http/multipartparser.py only catches ValueError:

except ValueError:  # Invalid header.
    continue

Since LookupError is not a subclass of ValueError, the exception propagates and results in a 500 Internal Server Error.

Steps to Reproduce

1. Create a simple upload view:

   from django.http import JsonResponse
   from django.views.decorators.csrf import csrf_exempt
   @csrf_exempt
   def upload(request):
       if request.method == "POST" and request.FILES:
           return JsonResponse({"filename": request.FILES["file"].name})
       return JsonResponse({"error": "No file"}, status=400)
   

2. Send a multipart request with a bogus encoding:

   import http.client
   boundary = "----PoC"
   body = (
       f"--{boundary}\r\n"
       f"Content-Disposition: form-data; name=\"file\"; "
       f"filename*=BOGUS''test%20file.txt\r\n"
       f"Content-Type: application/octet-stream\r\n"
       f"\r\n"
       f"content\r\n"
       f"--{boundary}--\r\n"
   )
   conn = http.client.HTTPConnection("localhost", 8000)
   conn.request("POST", "/upload/", body=body.encode(),
       headers={"Content-Type": f"multipart/form-data; boundary={boundary}"})
   print(conn.getresponse().status)  # Returns 500
   

The filename must contain at least one percent-encoded character (e.g., %20) for unquote() to invoke the codec.

Confirmed on Django 6.0.2, Python 3.14.

PR link: ​https://github.com/django/django/pull/20714

[sammiee5311]

PR: ​https://github.com/django/django/pull/20714

All tests pass (requests_tests — 115 tests) under SQLite, Python 3.14, macOS.