Form.files should be a MultiValueDict

[rolandcrosby-check]

Bug report

In #909, the BaseForm class's files attribute (and everything related to it, like the files argument to Widget.value_from_datadict) were updated to _FilesT, which is defined as Mapping[str, Iterable[File]]. Unless there are counterexamples in the Django codebase that I haven't seen, I'm pretty sure this can be more specific than Mapping - it appears to be a MultiValueDict (which is what this was typed as in previous versions of django-stubs).

Is it safe to assume that this is a MultiValueDict everywhere, or can users only rely on it being a Mapping?

[sterliakov]

(disclaimer: it was my commit)

Using Iterable was definitely a mistake, yes, it should have been Mapping[str, File].

I will revert this for now in #925 and use MultiValueDict[str, UploadedFile] for now again. I see the point that Mapping[str, Iterable[File]] is absolutely invalid (it doesn't return iterables as values) and Mapping[str, UploadedFile] doesn't work with multiple files upload (no .getlist method). However, there's problem with MultiValueDict[str, UploadedFile] too: it invalidates all test cases where we use MyForm({}, {'file_field': UploadedFile(...)}).

I think this case needs discussion and mainainer's final decision.

@sobolevn I'm sure you're one of maintainers, please resolve this or ping somebody else :)

[rolandcrosby-check]

@sterliakov thanks for taking a look (and for all the great enhancements you made!)