This method is used to manually set mime boundary string. If we don't call this method, the default behaviour is used - aka boundary is generated by https://godoc.org/mime/multipart package.
Also i have added unit tests to ensure it works as expected. And all old unit tests are passing.

Reason i want this method to be present, is because i have to deal with legacy report processing java application, and it hangs sometimes, when i send messages generated by your package.
As far as i understand how this java dinosaur works, it throws errors because boundary generated is 30 bytes long.

https://golang.org/src/mime/multipart/writer.go#L78

func randomBoundary() string {

	var buf [30]byte

	_, err := io.ReadFull(rand.Reader, buf[:])

	if err != nil {

		panic(err)

	}

	return fmt.Sprintf("%x", buf[:])

}

And it seems too big for java dinosaur code. Or maybe badly written regex in it... hard to say.
But if i could set boundary manually, you will make my day!
thanks