Current multipart option for multipart/related requests only works with buffers or string body.
When setting body to a stream (like a fs.createReadStream()) it silently fails and sends an empty part.

This would be a great addition to request to handle this.

As we don't know the stream final size this forces the use of chunk encoding, but it's not an issue for me.

In the meantime we could have a real error when the body type is not supported.