Make `filefieldbuffer.py` private #311

Job	Run time
py3.12 \| macos \| integration tests	15m 18s
mypy	3m 38s
py3.12 \| ubuntu \| integration tests	18m 12s
py3.12 \| macos \| unit tests	10m 51s
py3.12 \| ubuntu \| unit tests	13m 45s
py3.12 \| windows \| integration tests	32m 25s
py3.12 \| windows \| unit tests	18m 39s
py3.10 \| ubuntu \| unit tests	12m 47s
py3.10 \| ubuntu \| integration tests	18m 4s
py3.11 \| ubuntu \| unit tests	12m 41s
merge-test-artifacts	5s
	2h 36m 25s

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