txt,圖片等實現默認下載而不是打開圖片

在網頁上，如果我們的超鏈接的地址對應的是一個jpg文件,txt文件等，點擊鏈接時，瀏覽器默認的是打開這些文件而不是下載，那麼如何才能實現默認下載呢。
如果需要實現默認點擊彈出下載框,需要特殊處理
前端代碼:

function downloadFile(docUrl, fileName) {
        window.location.href = "<%=basePath%>ajax/downloadFile?url=" + docUrl + "&filename=" + fileName;
    }

<a href="javascript:void(0);" onclick="downloadFile('docUrl','fileName')"/>

後端代碼:

@GetMapping(produces = {"text/html;charset=UTF-8"})
    public void downloadFile(HttpServletRequest request, HttpServletResponse response,
        @RequestParam(value = "url") String urlStr, @RequestParam(value = "filename") String fileName)
        throws IOException
    {
        URL url = new URL(urlStr);
        response.setContentType("multipart/form-data");
        response.setHeader("Content-Disposition", "attachment;fileName=" + URLEncoder.encode(fileName, "UTF-8"));
        HttpURLConnection conn = null;
        InputStream inputStream = null;
        ServletOutputStream out = null;
        try
        {
            conn = (HttpURLConnection)url.openConnection();
            inputStream = conn.getInputStream();
            out = response.getOutputStream();
            IOUtils.copy(inputStream, out);
            out.flush();
        }
        catch (IOException e)
        {
            throw e;
        }
        finally
        {
            if (null != inputStream)
            {
                try
                {
                    inputStream.close();
                }
                catch (IOException e)
                {
                    throw e;
                }
            }
            if (null != out)
            {
                try
                {
                    out.close();
                }
                catch (IOException e)
                {
                    throw e;
                }
            }
            if (null != conn)
            {
                try
                {
                    conn.disconnect();
                }
                catch (Exception e)
                {
                    throw e;
                }
            }
        }
    }