ipfs api ignores content-transfer-encoding header

I tried `POST`ing a `multipart/form-data` request to the ipfs http api which contained a part with base64 encoded content:

```
--=====8252e0e51c6346e395185d392e7a7502=====
Content-Type: image/png
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-disposition: file; filename="sample/screenshot.png"

iVBORw0KGgoAAAANSUhEUgAAAGQAAAA8CAIAAAAfXYiZAAAABGdBTUEAALGPC/xhBQAAACBjSFJN
[...]
```

If IPFS isn't going go handle the `Content-transfer-encoding` header
then it should reject the request, rather than silently accepting it
and storing the base64 encoded content.

Start by tracing Kubo's HTTP API handling for multipart/form-data uploads and reproduce the request described in the issue. Check how the Content-Transfer-Encoding header is treated; done means the API no longer silently stores base64 text as file content and instead handles or rejects the encoding explicitly.

Written by the indexing model from the issue text.