Throw errors if file is not found or cannot be read
Add DirectFilePathResult, which redirects to accessible files or streams inaccessible files.
Add ability to clear / invalidate the cache
Add typedefs for different ActionResult permutations

(Future &&|| Outcome)
AcceptEither tests.
Add some unit tests, fixes for those tests.
Work-in-progress for upload handling. Complete typing for HttpRequest and HttpResponse.

Upload handling is still needing attention, please excuse the WIP comments.

Extra type hints were needed for mockatoo to mock correctly.