🐛 Applies strict type conversion for finalizing the multipart file

[AlexV525]

Fixes #2236

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
dio/lib/src/multipart_file.dart	🟢 88.57%	🟢 88.89%	🟢 0.32%
Overall Coverage	🟢 81.41%	🟢 81.28%	🔴 -0.13%

Minimum allowed coverage is 0%, this run produced 81.28%

[AlexV525]

If the only precise coverage diff is positive, how would the overall coverage diff become negative? :D

[kuhnroyal]

I have no idea...