R1:a7a068f84cd0

Correct two parameter strictness issues with file uploads

Summary:
Fixes T12531. Strictness fallout from adding typechecking in D17616.

- `chunkedHash` is not a real parameter, so the new typechecking was unhappy about it.
- `mime-type` no longer allows `null`.

Test Plan:
- Ran `arc upload --conduit-uri ... 12MB.zero` on a 12MB file full of zeroes.
- Before patch: badness, failure, fallback to one-shot uploads.
- After patch: success and glory.

Reviewers: chad

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