static Multipart of(String boundary, BodyPart... parts) {
        requireNonNull(parts, "parts");
        return of(boundary, ImmutableList.copyOf(parts));
    }
Returns a new {@link Multipart} with the specified {@code boundary} and {@link BodyPart}s.
core/src/main/java/com/linecorp/armeria/common/multipart/Multipart.java