Add default_to_mulitpart parameter

[Dreamsorcerer]

Fixes #6587.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.56%. Comparing base (9ab74e2) to head (fa634b4).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9335   +/-   ##
=======================================
  Coverage   98.56%   98.56%           
=======================================
  Files         107      107           
  Lines       34939    34944    +5     
  Branches     4145     4145           
=======================================
+ Hits        34438    34443    +5     
  Misses        334      334           
  Partials      167      167           

Flag	Coverage Δ
CI-GHA	98.44% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.11% <100.00%> (-0.01%)	⬇️
OS-Windows	96.52% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.79% <100.00%> (-0.01%)	⬇️
Py-3.10.11	97.68% <100.00%> (-0.01%)	⬇️
Py-3.10.15	97.61% <100.00%> (+<0.01%)	⬆️
Py-3.11.10	97.67% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.74% <100.00%> (+<0.01%)	⬆️
Py-3.12.6	98.16% <100.00%> (+<0.01%)	⬆️
Py-3.13.0-rc.2	98.15% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	97.57% <100.00%> (-0.01%)	⬇️
Py-3.9.20	97.51% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.16	97.13% <100.00%> (-0.01%)	⬇️
VM-macos	97.79% <100.00%> (-0.01%)	⬇️
VM-ubuntu	98.11% <100.00%> (-0.01%)	⬇️
VM-windows	96.52% <100.00%> (+<0.01%)	⬆️

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[patchback]

Backport to 3.11: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply d9a9368 on top of patchback/backports/3.11/d9a9368f0923e1447231880075271e7fe38683d8/pr-9335

Backporting merged PR #9335 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.11/d9a9368f0923e1447231880075271e7fe38683d8/pr-9335 upstream/3.11

4. Now, cherry-pick PR Add default_to_mulitpart parameter #9335 contents into that branch:

   $ git cherry-pick -x d9a9368f0923e1447231880075271e7fe38683d8

   If it'll yell at you with something like fatal: Commit d9a9368f0923e1447231880075271e7fe38683d8 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x d9a9368f0923e1447231880075271e7fe38683d8

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Add default_to_mulitpart parameter #9335 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.11/d9a9368f0923e1447231880075271e7fe38683d8/pr-9335

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

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