private static MediaType defaultContentType(@Nullable ContentDisposition contentDisposition) {
        if (contentDisposition != null && contentDisposition.filename() != null) {
            return MediaType.OCTET_STREAM;
        } else {
            return MediaType.PLAIN_TEXT;
        }
    }
Returns the default {@code "content-type"} header value:
{@link MediaType#OCTET_STREAM} if the {@code "content-disposition"} header is present with
a non empty value, otherwise {@link MediaType#PLAIN_TEXT}.

@see <a href="https://datatracker.ietf.org/doc/html/rfc7578#section-4.4">RFC-7578</a>
core/src/main/java/com/linecorp/armeria/common/multipart/BodyPartBuilder.java