Added missing newline.
Refactored the way Chunks are reassembled.

Removed a rather hacky way of determine which of the chunks to use as a base, and replaced it with an Iterator as mentioned in #21.
Use the correct typehint for Iterators.
Remove the prefix after assembling the last chunk.

Until now, the reassembled file shared the name with the very first chunk uploaded.
It had the form {prefix}_{filename}. This commit fixes this issue and addresses #21 with it.