Request attribute upload handlers is immutable when method is POST

[matsony_15]

Hi, this is copy of #35447. I`m trying to use custom upload handler as its mentioned ​here (​https://docs.djangoproject.com/en/5.0/topics/http/file-uploads//), but when request method == POST upload_handlers atribute of request has type immutable list and throws an error. I created a test repo and provided some additional info in README.md here ​​https://github.com/Smoooky/custom_upload_handler

[Sarah Boyce]

This feels like a duplicate of #26049

In the docs there is a ​note

You can only modify upload handlers before accessing request.POST or request.FILES – it doesn’t make sense to change upload handlers after upload handling has already started. If you try to modify request.upload_handlers after reading from request.POST or request.FILES Django will throw an error.

I suspect the issue is that you're accessing request.POST in the init of ProgressBarUploadHandler

If you want to discuss this further, I recommend taking this to the ​forum 👍