feat: allow custom mode for Artifact.new_file

[borisdayma]

Same as https://github.com/wandb/client/pull/1202

[coveralls]

Pull Request Test Coverage Report for Build a48d1dbf-6ea3-4452-9c73-8500b30f9d25

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+5.0%) to 64.937%

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Totals
Change from base Build c77330b6-633c-4714-bbc7-aeedb063d688:	5.0%
Covered Lines:	12325
Relevant Lines:	18980

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💛 - Coveralls

[borisdayma]

Note: I can't access the CI, maybe it should be public?