How do I send regular post key/value pairs with a file upload in android

I have a function for uploading photos via post. I can upload the file fine, but not sure how to go about adding more key/value pairs to POST, basically I need to accopmany an API key and Session key along with the file data, the method looks like this.

public ContainerData submitPhoto(FileInputStream fileInputStream, String sessionKey) {

    try {

        URL url = new URL(API_URL);

        HttpURLConnection conn = null;

        if (url.getProtocol().toLowerCase().equals("https")) {
            trustAllHosts();
            HttpsURLConnection https = (HttpsURLConnection) url.openConnection();
            https.setHostnameVerifier(DO_NOT_VERIFY);
            conn = https;
        } else {
            conn = (HttpURLConnection) url.openConnection();
        }


        String lineEnd = "\r\n";
        String twoHyphens = "--";
        String boundary = "*****";

        // HttpURLConnection conn = (HttpURLConnection)
        // connectURL.开发者_StackOverflow社区openConnection();

        // Allow Inputs
        conn.setDoInput(true);

        // Allow Outputs
        conn.setDoOutput(true);

        // Don't use a cached copy.
        conn.setUseCaches(false);

        // Use a post method.
        conn.setRequestMethod("POST");

        conn.setRequestProperty("Connection", "Keep-Alive");

        conn.setRequestProperty("Content-Type", "multipart/form-data;boundary=" + boundary);

        DataOutputStream dos = new DataOutputStream(conn.getOutputStream());

        dos.writeBytes(twoHyphens + boundary + lineEnd);
        dos.writeBytes("Content-Disposition: form-data; name=\"uploadedfile\";filename=\"" + "file.png" + "\"" + lineEnd);
        dos.writeBytes(lineEnd);

        // create a buffer of maximum size

        int bytesAvailable = fileInputStream.available();
        int maxBufferSize = 1028;
        int bufferSize = Math.min(bytesAvailable, maxBufferSize);
        byte[] buffer = new byte[bufferSize];

        // read file and write it into form...

        int bytesRead = fileInputStream.read(buffer, 0, bufferSize);
        while (bytesRead > 0) {
            dos.write(buffer, 0, bufferSize);
            bytesAvailable = fileInputStream.available();
            bufferSize = Math.min(bytesAvailable, maxBufferSize);
            bytesRead = fileInputStream.read(buffer, 0, bufferSize);
            /*
             * dos.writeBytes(lineEnd); dos.writeBytes(twoHyphens + boundary
             * + twoHyphens + lineEnd);
             */
        }

        // send multipart form data necesssary after file data...
        dos.writeBytes(lineEnd);
        dos.writeBytes(twoHyphens + boundary + twoHyphens + lineEnd);
        dos.flush();
        // Log.d(TAG, "  dos5: " + dos.toString());
        InputStream is = conn.getInputStream();
        // retrieve the response from server
        int ch;

        StringBuffer b = new StringBuffer();
        while ((ch = is.read()) != -1) {
            b.append((char) ch);
        }
        String stringResponse = b.toString();
        // Log.d(TAG, "http response for upload" + s);
        dos.close();

        Gson gson = new GsonBuilder().serializeNulls().create();
        responseObject = gson.fromJson(stringResponse,ContainerData.class);

        JSONObject data = new JSONObject(stringResponse);
        String dataResponse = data.getString("data");
        responseObject.setDataString(dataResponse);


    } catch (JSONException e) {
        e.printStackTrace();
    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return responseObject;

}

RFC 1867 specifies the format. From the link:

Content-type: multipart/form-data, boundary=AaB03x

--AaB03x
content-disposition: form-data; name="field1"

Joe Blow
--AaB03x
content-disposition: form-data; name="pics"; filename="file1.txt"
Content-Type: text/plain

... contents of file1.txt ...
--AaB03x--

Two additional issues with your code:

1) I find recreating the buffer at each iteration cumbersome and slow. Just dim it at the maximum size before the loop and reuse it.

2) Also, sending directly the data bytes may be risky (what if two bytes of the picture have the same value than your separator? I would advise using base64 encoding.