[Design] Pre-process Input Stream

[gregdhill]

Assuming the architecture as implemented in #90, we will stream chunks of raw data after a header which contains metadata concerning the file as uploaded. Currently, this contains some generic fields such as Type to indicate a file's MIME type. If we were to store this header last, after processing the entire input stream, we could add some interesting information such as the total data written or a rolling hash. In order to still serve a partial 'read' (where we return the header without the body) we would expect the client to re-order the references.