Fix misleading file extension for partition statistics (#3472) #8829

Job	Run time
integration-test	10m 34s
integration-test-s3	1m 13s
cibw-dev-env-smoke-test	23s
lint-and-unit-test (3.11)	3m 2s
lint-and-unit-test (3.10)	3m 29s
lint-and-unit-test (3.12)	3m 23s
integration-test-gcs	1m 14s
integration-test-adls	1m 25s
lint-and-unit-test (3.13)	2m 55s
lint-and-unit-test (3.14)	2m 44s
integration-coverage-report	27s
	30m 49s

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