解决方案:
在SpringMVC里添加如下代码即可。
<!--ByteArray的消息转换器-->
<bean id="byteArrayHttpMessageConverter" class="org.springframework.http.converter.ByteArrayHttpMessageConverter"/>
<bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter">
<property name="messageConverters">
<util:list id="beanList">
<ref bean="byteArrayHttpMessageConverter"/>
</util:list>
</property>
</bean>
===================2019.01.11看到好文章======================
SpringMVC 文件批量上传:https://blog.youkuaiyun.com/qq_33422712/article/details/79147094
文件批量下载:https://blog.youkuaiyun.com/qq_33422712/article/details/79142350