(stable) Correct two parameter strictness issues with file uploads
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.
- 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.
Maniphest Tasks: T12531
Differential Revision: https://secure.phabricator.com/D17651