解决了。得用原始的请求。

public static String uploadmedia(String authorizerAccessToken, MultipartFile mediaFile) {
    try {
        URL urlObj = new URL("https://api.weixin.qq.com/wxa/uploadmedia?access_token=" + authorizerAccessToken);
        HttpURLConnection conn = (HttpURLConnection)urlObj.openConnection();
        conn.setRequestMethod("POST");
        conn.setDoInput(true);
        conn.setDoOutput(true);
        conn.setUseCaches(false);
        conn.setRequestProperty("Connection", "Keep-Alive");
        conn.setRequestProperty("Charset", "UTF-8");
        String BOUNDARY = "----------" + System.currentTimeMillis();
        conn.setRequestProperty("Content-Type", "multipart/form-data;boundary=" + BOUNDARY);
        StringBuilder sb = new StringBuilder();
        sb.append("--");
        sb.append(BOUNDARY);
        sb.append("\r\n");
        sb.append("Content-Disposition:form-data;name=\"media\";filename=\"" + mediaFile.getOriginalFilename() + "\";filelength=\"" + mediaFile.getSize() + "\"\r\n");
        sb.append("Content-Type:application/octet-stream\r\n\r\n");
        byte[] head = sb.toString().getBytes("utf-8");
        OutputStream out = new DataOutputStream(conn.getOutputStream());
        out.write(head);
        out.write(mediaFile.getBytes());
        byte[] foot = ("\r\n--" + BOUNDARY + "--\r\n").getBytes("utf-8");
        out.write(foot);
        out.flush();
        out.close();
        StringBuffer buffer = new StringBuffer();
        BufferedReader reader = null;
        String result = null;

        try {
            reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));
            String line = null;

            while((line = reader.readLine()) != null) {
                buffer.append(line);
            }

            if (result == null) {
                result = buffer.toString();
            }
        } catch (IOException var17) {
            var17.printStackTrace();
        } finally {
            reader.close();
        }

        return result;
    } catch (IOException var19) {
        logger.error(BizLog.buildLogInfo("uploadmedia: " + ExceptionUtils.getStackTrace(var19)));
        return null;
    }
}

没有正确传递文件内容吧？