Make `filefieldbuffer.py` private #310

Job	Run time
py3.12 \| macos \| unit tests	41s
mypy	42s
py3.12 \| macos \| integration tests	41s
py3.12 \| ubuntu \| unit tests	40s
py3.12 \| ubuntu \| integration tests	1s
py3.12 \| windows \| integration tests	34s
py3.12 \| windows \| unit tests	36s
py3.10 \| ubuntu \| integration tests	40s
py3.10 \| ubuntu \| unit tests	40s
py3.11 \| ubuntu \| unit tests	39s
merge-test-artifacts	1s
	5m 55s

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