* Unsatisfied conditions will return AccessDenied instead of MissingFields
* Require form-field `file` in POST policy and make `filename` an optional attribute
* S3 feature: Replace in Key by filename attribute passed in multipart
Signature calculation has now moved out from being a package to
top-level as a layered mechanism.
In case of payload calculation with body, go-routines are initiated
to simultaneously write and calculate shasum. Errors are sent
over the writer so that the lower layer removes the temporary files
properly.
