前台使用的是vue，用el-upload上传附件到后台后，接到的是null，F12控制台发现文件已经获取，下面是解决方式
创建个config，让项目启动时被spring注入，就可以接收到文件信息了

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.multipart.MultipartResolver;
import org.springframework.web.multipart.commons.CommonsMultipartResolver;

@Configuration
public class UploadConfig {
    //显示声明CommonsMultipartResolver为mutipartResolver
    @Bean(name = "multipartResolver")
    public MultipartResolver multipartResolver() {
        CommonsMultipartResolver resolver = new CommonsMultipartResolver();
        resolver.setDefaultEncoding("UTF-8");
        //resolveLazily属性启用是为了推迟文件解析，以在在UploadAction中捕获文件大小异常
        resolver.setResolveLazily(true);
        resolver.setMaxInMemorySize(40960);
        //上传文件大小 50M 50*1024*1024
        resolver.setMaxUploadSize(50 * 1024 * 1024);
        return resolver;
    }

}