title: Retrofit form-data POST not return JSON
body: <p>I try this POST in a server using Apache HttpClient and working perfectly:</p>
  
  <pre><code>            HttpClient httpclient = new DefaultHttpClient();
              HttpPost httppost = new HttpPost(BASE_URL + "access.php");
              List&lt;NameValuePair&gt; nameValuePairs = new ArrayList&lt;NameValuePair&gt;(3);
              nameValuePairs.add(new BasicNameValuePair(PARAMETER_COD, mAccessCode));
              nameValuePairs.add(new BasicNameValuePair(PARAMETER_USU, mUser));
              nameValuePairs.add(new BasicNameValuePair(PARAMETER_PAS, mPassword));
              httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
              try {
                  HttpResponse response = httpclient.execute(httppost);
                  //JSON RESPONSE
                  String op = EntityUtils.toString(response.getEntity(), "UTF-8");
  
              } catch (IOException e) {
                  e.printStackTrace();
              }
              //reset the message text field
          } catch (IOException e) {
              e.printStackTrace();
          }
  </code></pre>
  
  <p>But I tried make usin Retrofit and not working:</p>
  
  <pre><code>@Multipart
  @POST("access.php")
  Call&lt;String&gt; authenticate(@Part("cod") String cod, @Part("usu") String usu, @Part("pas") String pas);
  </code></pre>
  
  <p>private ApiInterface getInterfaceService() {</p>
  
  <pre><code>    Gson gson = new GsonBuilder()
              .setLenient()
              .create();
      Retrofit retrofit = new Retrofit.Builder()
              .baseUrl(BASE_URL)
              .addConverterFactory(GsonConverterFactory.create(gson))
              .build();
  
      final ApiInterface mInterfaceService = retrofit.create(ApiInterface.class);
      return mInterfaceService;
  }
  
  private void loginProcessWithRetrofit(final User user){
  
      ApiInterface mApiService = this.getInterfaceService();
      Call&lt;String&gt; mService = mApiService.authenticate(String.valueOf(user.cod),user.pas,user.usu);
      mService.enqueue(new Callback&lt;String&gt;() {
          @Override
          public void onResponse(Call&lt;String&gt; call, Response&lt;String&gt; response) {
  
              //String returnedResponse = mLoginObject.isLogin;
              Toast.makeText(LoginActivity.this, "Returned " + response.body(), Toast.LENGTH_LONG).show();
              showProgress(false);
          }
  
          @Override
          public void onFailure(Call&lt;String&gt; call, Throwable t) {
              call.cancel();
              Toast.makeText(LoginActivity.this, "Please check your network connection and internet permission", Toast.LENGTH_LONG).show();
              Log.e("Retrofit",t.toString());
          }
      });
  }
  </code></pre>
  
  <p>Also try this other post: </p>
  
  <pre><code>@POST("access.php")
  Call&lt;String&gt; authenticate(@Body User user);
  </code></pre>
  
  <p>What is the correct form to conver my HttpPost to Retrofit. 
  Thanks a lot.
   Regards</p>