Add multipart_parser_class attribute to HttpRequest for pluggable multipart parsing

[Farhan Ali]

Problem

Django's parse_file_upload() hardcodes MultiPartParser, making it hard
to swap in faster implementations.

Proposed Solution

As discussed with the steering council and Carlton Gibson on the forum:

1. Add multipart_parser_class class attribute to HttpRequest (defaulting to MultiPartParser)

2. Modify parse_file_upload() to use self.multipart_parser_class instead of direct import

3. Ensure uniform parser interface: __init__(META, input_data, upload_handlers, encoding) with parse() returning (POST, FILES)

Usage

Users can subclass HttpRequest and override multipart_parser_class to use
faster parsers.

Benefits

• Immediate performance improvements for those who need them
• Stepping stone toward DEP 17 ​https://github.com/django/deps/pull/88 (pluggable content type parsers)
• Minimal change
• Follows proven pattern from Django REST Framework's parser_classes

Forum Discussion

​https://github.com/django/new-features/issues/105

[Natalia Bidart]

Thanks for opening the ticket and for following the prior discussion!

The DRF comparison is useful as prior art, but this does not add view-level parser_classes, keeping the scope intentionally small. Configurability via a custom HttpRequest subclass is sufficient at this stage and keeps the change minimal.

Proceeding with a PR along these lines sounds good, looking forward to that!

[Farhan Ali]

Opened a PR ​https://github.com/django/django/pull/20498. But it will need a guide in the docs, I think how to use this feature properly.

[Jacob Walls]

Don't want to increase scope here, but just left one question to ensure we protect the smoothest possible adoption path for the pending content-type DEP.

[Jacob Walls]

I think this is ready. I'll leave it RFC for a week or two given that I just pinged a couple folks, but I won't let it die on the vine.

[Jacob Walls]

Still on the verge of merge, one tweak needed on the exception class.

[Jacob Walls <jacobtylerwalls@…>]

In 7732f94:

Fixed #36841 -- Made multipart parser class pluggable on HttpRequest.