Actually make chunking with multipart mode work.

* Fix send file-specific POST data.
* Fix UploadSucceed server response data pass-through.
* Fix file progress indication.
Oops! Fixed obvious mistake.
Fix multipart upload progress indication.

ProgressEvents only fire right after a chunk is completed and indicate the
length of the RECEIVED data. Ignore them, and calculate progress in
chunkComplete() instead.
Merge pull request #3 from bertm/master

Fixing the multipart / chunking mode