public BodyPartBuilder content(Publisher<? extends HttpData> publisher) {
        requireNonNull(publisher, "publisher");
        contentsBuilder.add(publisher);
        return this;
    }
Adds a new body part backed by the specified {@link Publisher}.
@param publisher publisher for the part content
core/src/main/java/com/linecorp/armeria/common/multipart/BodyPartBuilder.java