Improve typings in multipart

[kornicameister]

Adjusts typings to match the documentation.

No, those were just typings improvements.
I've changed the code in one place but those changes were limited to:

• using (IMHO) more appropriate looping technique
• accounting for changed typins

#3621

No

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[kornicameister]

About non changing the user behavior. There's one place that I am not particularly sure of. It is the place around assert field_content_type is not None. I suppose that's having something with the typing. Unfortunately the FileField expects is set, so I though that if anything goes wrong there and content_type is actually None is is best to throw the processing out.

[kornicameister]

@asvetlov is this still needed by any means?

[asvetlov]

1. The PR doesn't pass CI tests. I don't review such PRs usually.
2. Please keep while loop in web_request.py. I agree that async for would be better but the change introduces a big hard-to-review diff without strong reason.

[kornicameister]

Ok, @asvetlov that sounds like a plan. I will close this PR and first resolve the typings issue and then make a PR with async for given that it is doable to these changes in separation. I don't remember what was the deal with adding async for, but for sure I have done that for some reason.

Anyway, I will try to deal with typings first.

PS. sorry for not dealing with CI first.

[kornicameister]

Closing now