static Multipart from(String boundary, Publisher<? extends HttpData> contents, ByteBufAllocator alloc) {
        requireNonNull(boundary, "boundary");
        requireNonNull(contents, "contents");
        requireNonNull(alloc, "alloc");
        final MultipartDecoder decoder = new MultipartDecoder(StreamMessage.of(contents), boundary, alloc);
        return of(boundary, decoder);
    }
Returns a decoded {@link Multipart} from the specified {@code boundary},
{@link Publisher} of {@link HttpData} and {@link ByteBufAllocator}.
core/src/main/java/com/linecorp/armeria/common/multipart/Multipart.java