private void postSingleFile() {
    new Thread(new Runnable() {
        @Override
        public void run() {
            BufferedInputStream bis   = null;
            BufferedReader br         = null;
            OutputStream outputStream = null;
            try {
                @SuppressLint("SdCardPath") File filePng = new File("/sdcard/Download/browser/e1045e8079daa721f7309b4a61fdc1cb.jpg");
                String fileKey = "file";
                String fileName = filePng.getName();
                String fileType = "image/jpeg";
                URL url = new URL(BASE_URL + "/file/upload");
                HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
                httpURLConnection.setConnectTimeout(5000);
                httpURLConnection.setRequestMethod("POST");

                httpURLConnection.setRequestProperty("User-Agent", "Android/" + Build.VERSION.SDK_INT);
                httpURLConnection.setRequestProperty("Accept", "*/*");
                httpURLConnection.setUseCaches(false);
                httpURLConnection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + BOUNDARY);
                httpURLConnection.setDoInput(true);
                httpURLConnection.setDoOutput(true);
                httpURLConnection.connect();

                //准备数据
                StringBuilder headerInfo = new StringBuilder();
                headerInfo.append("--");
                headerInfo.append(BOUNDARY);
                headerInfo.append("\r\n");
                headerInfo.append("Content-Disposition: form-data; name=\"" + fileKey + "\"; filename=\"" + fileName + "\"");
                headerInfo.append("\r\n");
                headerInfo.append("Content-Type: " + fileType);
                headerInfo.append("\r\n");
                headerInfo.append("\r\n");
                byte[] headerInfoBytes = headerInfo.toString().getBytes("UTF-8");

                //写入头部信息
                outputStream = httpURLConnection.getOutputStream();
                outputStream.write(headerInfoBytes);
                
                //写入文件
                FileInputStream fis = new FileInputStream(filePng);
                bis = new BufferedInputStream(fis);
                byte[] buffer = new byte[1024];
                int len;
                while((len = bis.read(buffer,0,buffer.length)) != -1){
                    outputStream.write(buffer,0,len);
                }

                //写入尾部信息
                StringBuilder footerInfo = new StringBuilder();
                footerInfo.append("\r\n");
                footerInfo.append("--");
                footerInfo.append(BOUNDARY);
                footerInfo.append("--");
                footerInfo.append("\r\n");
                footerInfo.append("\r\n");
                outputStream.write(footerInfo.toString().getBytes("UTF-8"));
                outputStream.flush();

                //获取返回结果
                int responseCode = httpURLConnection.getResponseCode();
                Log.v(TAG,"responseCode --------> " + responseCode);
                if(responseCode == 200){
                    InputStream is = httpURLConnection.getInputStream();
                    br = new BufferedReader(new InputStreamReader(is));
                    String dataReturned = br.readLine();
                    Log.v(TAG,dataReturned);
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
            finally {
                if(outputStream != null){
                    try {
                        outputStream.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
                if(bis != null){
                    try {
                        bis.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
                if(br != null){
                    try {
                        br.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }

        }
    }).start();
}

流程应该也没问题，不知道为什么出不来log，下面是点击了按钮之后的log