参考： http://www.layui.com/doc/modules/upload.html

<1> 文件上传(以下函数必须要在js文件加载时执行)
upload.render({
elem: '#id',
url: '/api/upload/',
before: function(obj){ //obj参数包含的信息，跟 choose回调完全一致，可参见上文。
layer.load(); //上传loading
},
done: function(res, index, upload){
layer.closeAll('loading'); //关闭loading
},
error: function(index, upload){
layer.closeAll('loading'); //关闭loading
}
});
<2> 文件下载 参考：https://memorynotfound.com/spring-mvc-download-file-examples/

package com.memorynotfound.controller;

import org.springframework.core.io.FileSystemResource;
import org.springframework.core.io.Resource;
import org.springframework.http.HttpEntity;
import org.springframework.http.HttpHeaders;
import org.springframework.http.MediaType;
import org.springframework.stereotype.Controller;
import org.springframework.util.FileCopyUtils;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;
import javax.servlet.http.HttpServletResponse;
import java.io.*;

@Controller
@RequestMapping("/download")
public class DownloadController {

private static final String FILE\_PATH = "/tmp/example.pdf";  
private static final String APPLICATION\_PDF = "application/pdf";

@RequestMapping(value = "/a", method = RequestMethod.GET, produces = APPLICATION\_PDF)  
public @ResponseBody void downloadA(HttpServletResponse response) throws IOException {  
    File file = getFile();  
    InputStream in = new FileInputStream(file);

    response.setContentType(APPLICATION\_PDF);  
    response.setHeader("Content-Disposition", "attachment; filename=" + file.getName());  
    response.setHeader("Content-Length", String.valueOf(file.length()));  
    FileCopyUtils.copy(in, response.getOutputStream());  
}

@RequestMapping(value = "/b", method = RequestMethod.GET, produces = APPLICATION\_PDF)  
public @ResponseBody HttpEntity<byte\[\]> downloadB() throws IOException {  
    File file = getFile();  
    byte\[\] document = FileCopyUtils.copyToByteArray(file);

    HttpHeaders header = new HttpHeaders();  
    header.setContentType(new MediaType("application", "pdf"));  
    header.set("Content-Disposition", "inline; filename=" + file.getName());  
    header.setContentLength(document.length);

    return new HttpEntity<byte\[\]>(document, header);  
}

@RequestMapping(value = "/c", method = RequestMethod.GET, produces = APPLICATION\_PDF)  
public @ResponseBody Resource downloadC(HttpServletResponse response) throws FileNotFoundException {  
    File file = getFile();  
    response.setContentType(APPLICATION\_PDF);  
    response.setHeader("Content-Disposition", "inline; filename=" + file.getName());  
    response.setHeader("Content-Length", String.valueOf(file.length()));  
    return new FileSystemResource(file);  
}

private File getFile() throws FileNotFoundException {  
    File file = new File(FILE\_PATH);  
    if (!file.exists()){  
        throw new FileNotFoundException("file with path: " + FILE\_PATH + " was not found.");  
    }  
    return file;  
}

出现的问题：
1. 上传后，出现文件下载框（一般为ie下），那么你需要在服务端对response的header设置 Content-Type: text/html
　　response.addHeader("Content-Type","text/html");
2. 如果上传后，文件名称回显为乱码
　　response.addHeader("Content-Type","text/html;charset=utf-8");