您需要设置 Content-Type 标头，并在 MultipartEntityBuilder 中生成正确的边界。修改您的发件人处理器以设置此标头。

You need to set Content-Type header with correct boundary, which is generated in MultipartEntityBuilder. Modify your sender processor to set this header.

HttpEntity resultEntity = multipartEntityBuilder.build();
exchange.getOut().setHeader(Exchange.CONTENT_TYPE, resultEntity.getContentType().getValue());
exchange.getOut().setBody(resultEntity.getContent());