Ecco un po' di codice per fare una POST HTTP di un file
codice:
        String bndry = "AaB03x";
        String paramName = "file";
        String fileName = "filename.txt";
        HttpURLConnection httpcon = (HttpURLConnection) ((new URL(url).openConnection()));
        httpcon.setDoOutput(true);
        httpcon.setRequestMethod("POST");
        httpcon.setRequestProperty("Content-type", "multipart/form-data, boundary=" +bndry);
        httpcon.connect();
        File file = new File(docPath);
        FileInputStream is = new FileInputStream(file);
        OutputStream os = httpcon.getOutputStream();
        String disptn = "--" +bndry
+"\r\ncontent-disposition: form-data; name=\"" +paramName
+"\"; filename=\"" +fileName 
+"\"\r\nContent-Type: text/plain\r\n\r\n";
        os.write(disptn.getBytes());


        byte[] buffer = new byte[4096];
        int bytes_read;
        while((bytes_read = is.read(buffer)) != -1) {
            os.write(buffer, 0, bytes_read);
        }

        String boundar = "\r\n--" +bndry +"--";
        os.write(boundar.getBytes()); 

        os.flush();
        os.close();
        is.close();