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 97.39%. Comparing base (270ae9c) to head (de19957).

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             @@
##              3.9    #8302      +/-   ##
==========================================
- Coverage   97.41%   97.39%   -0.03%     
==========================================
  Files         108      108              
  Lines       32820    32818       -2     
  Branches     3879     3889      +10     
==========================================
- Hits        31973    31963      -10     
- Misses        641      645       +4     
- Partials      206      210       +4     

Flag	Coverage Δ
CI-GHA	97.30% <93.05%> (-0.03%)	⬇️
OS-Linux	96.98% <93.05%> (-0.03%)	⬇️
OS-Windows	94.58% <93.05%> (-0.03%)	⬇️
OS-macOS	96.70% <93.05%> (-0.02%)	⬇️
Py-3.10.11	94.40% <93.05%> (-0.02%)	⬇️
Py-3.10.14	96.76% <93.05%> (-0.03%)	⬇️
Py-3.11.8	97.02% <93.05%> (-0.02%)	⬇️
Py-3.12.2	97.06% <93.05%> (-0.04%)	⬇️
Py-3.8.10	94.37% <93.05%> (-0.03%)	⬇️
Py-3.8.18	96.70% <93.05%> (-0.03%)	⬇️
Py-3.9.13	94.38% <93.05%> (-0.03%)	⬇️
Py-3.9.19	96.74% <93.05%> (-0.02%)	⬇️
Py-pypy7.3.15	96.29% <93.05%> (-0.03%)	⬇️
VM-macos	96.70% <93.05%> (-0.02%)	⬇️
VM-ubuntu	96.98% <93.05%> (-0.03%)	⬇️
VM-windows	94.58% <93.05%> (-0.03%)	⬇️

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