Make `filefieldbuffer.py` private #442

Job	Run time
mypy	3m 45s
py3.13 \| macos \| unit tests	9m 14s
py3.13 \| macos \| integration tests	13m 40s
py3.13 \| ubuntu \| unit tests	13m 17s
py3.13 \| ubuntu \| integration tests	12m 5s
py3.13 \| windows \| unit tests	21m 38s
py3.13 \| windows \| integration tests	19m 3s
py3.10 \| ubuntu \| unit tests	12m 58s
py3.10 \| ubuntu \| integration tests	11m 53s
py3.11 \| ubuntu \| unit tests	2m 40s
py3.12 \| ubuntu \| unit tests	2m 43s
merge-test-artifacts	0s
	2h 2m 56s

Provide feedback