Fix handling of multipart/form-data (#8280)

[Dreamsorcerer]

https://datatracker.ietf.org/doc/html/rfc7578
(cherry picked from commit 7d0be3f)

[codecov]

Codecov Report

Attention: Patch coverage is 93.05556% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 97.48%. Comparing base (d15f07c) to head (2fdcf49).

Files	Patch %	Lines
aiohttp/multipart.py	91.11%	2 Missing and 2 partials ⚠️
aiohttp/formdata.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             3.10    #8301      +/-   ##
==========================================
- Coverage   97.51%   97.48%   -0.03%     
==========================================
  Files         108      108              
  Lines       33270    33268       -2     
  Branches     3963     3973      +10     
==========================================
- Hits        32442    32432      -10     
- Misses        623      627       +4     
- Partials      205      209       +4     

Flag	Coverage Δ
CI-GHA	97.39% <93.05%> (-0.03%)	⬇️
OS-Linux	97.06% <93.05%> (-0.03%)	⬇️
OS-Windows	94.70% <93.05%> (-0.03%)	⬇️
OS-macOS	96.79% <93.05%> (-0.02%)	⬇️
Py-3.10.11	94.52% <93.05%> (-0.02%)	⬇️
Py-3.10.14	96.85% <93.05%> (-0.02%)	⬇️
Py-3.11.8	97.05% <93.05%> (-0.03%)	⬇️
Py-3.12.2	97.16% <93.05%> (-0.03%)	⬇️
Py-3.8.10	94.45% <93.05%> (-0.04%)	⬇️
Py-3.8.18	96.70% <93.05%> (-0.03%)	⬇️
Py-3.9.13	94.51% <93.05%> (-0.02%)	⬇️
Py-3.9.19	96.84% <93.05%> (-0.03%)	⬇️
Py-pypy7.3.15	96.36% <93.05%> (-0.03%)	⬇️
VM-macos	96.79% <93.05%> (-0.02%)	⬇️
VM-ubuntu	97.06% <93.05%> (-0.03%)	⬇️
VM-windows	94.70% <93.05%> (-0.03%)	⬇️

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