[swift6] Don't encode Data as base64 in multipart request

[x-sheep]

This will make sure Data is not encoded as base64 unless it's necessary for the request, i.e. it's part of the querystring or headers. A FormData request should always preserve the data as-is, to keep the same functionality as passing in a URL.

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[4brunu]

Hi @x-sheep, thanks for creating this PR.
Does this PR fixes this issue #21444?
Thanks

[x-sheep]

@4brunu It should fix that issue, but right now my uploads still fail because they don't have a filename or content type.