Prevent Addition of Extra Newline in Binary File Uploads

[mohamedlmoutaouakil]

The change in this PR addresses the problem of wrong parsing of multipart request body. Specifically using a wrong index of the last newline in the file data in the MultipartDecoder.

The problem is passing index shifted data to last_newline method and using the index returned by this method directly on the original non-index-shifted data.

• fixes Extra Newline Added to Binary File Upload #2761

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[raulpuric]

Can confirm this fixed my bugs. Thanks @mohamedlmoutaouakil

[pgjones]

Closed in favor of #2763