Report each file part's Content-Transport-Encoding as `encoding`

Otherwise the user does not get a chance to apply a decoding filter.  The
alternative might be to directly handle the various encodings directly
within busboy.

I was able to test this against a (large) set of base64-encoded parts, without a binary error.  I used https://github.com/mazira/base64-stream to decode the ingress streams.  Note that this PR does not add a dependency to busboy, as the way to decode encoded parts is left to the developer.