(stable) 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

Subscribers: joshuaspence

Maniphest Tasks: T12531

Differential Revision: https://secure.phabricator.com/D17651