Make `filefieldbuffer.py` private #309

Job	Run time
py3.12 \| macos \| integration tests	7m 35s
py3.12 \| ubuntu \| integration tests	7m 31s
mypy	3m 53s
py3.12 \| windows \| integration tests	2m 15s
py3.12 \| macos \| unit tests	7m 35s
py3.10 \| ubuntu \| integration tests	7m 31s
py3.12 \| ubuntu \| unit tests	7m 33s
py3.12 \| windows \| unit tests	7m 23s
py3.10 \| ubuntu \| unit tests	7m 33s
py3.11 \| ubuntu \| unit tests	7m 34s
merge-test-artifacts	1s
	1h 6m 24s

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