Add getContentType(...) and getDefaultHttpClient() method.

app/src/main/java/chat/rocket/android/helper/OkHttpHelper.java

 package chat.rocket.android.helper;
 
 import android.content.Context;
+import android.support.annotation.NonNull;
 import com.facebook.stetho.okhttp3.StethoInterceptor;
 
+import java.io.IOException;
 import java.util.concurrent.TimeUnit;
 import chat.rocket.android.RocketChatCache;
 import chat.rocket.android.api.rest.CookieInterceptor;
 import chat.rocket.android.api.rest.DefaultCookieProvider;
+import okhttp3.Call;
+import okhttp3.Callback;
 import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.Response;
 
 /**
  * Helper class for OkHttp client.
  */
 public class OkHttpHelper {
+  private static OkHttpClient defaultHttpClient;
   private static OkHttpClient httpClientForUploadFile;
   private static OkHttpClient httpClientForDownloadFile;
   private static OkHttpClient httpClientForWS;
+  private static String contentType;
+
+  public static OkHttpClient getDefaultHttpClient() {
+    if (defaultHttpClient == null) {
+      defaultHttpClient = new OkHttpClient();
+    }
+    return defaultHttpClient;
+  }
 
   public static OkHttpClient getClientForDownloadFile(Context context) {
     if (httpClientForDownloadFile == null) {
@@ -48,4 +63,28 @@ public class OkHttpHelper {
     }
     return httpClientForWS;
   }
+
+  public static String getContentType(String uri) {
+    Request request = new Request.Builder()
+        .url(uri)
+        .head()
+        .build();
+    getDefaultHttpClient().newCall(request).enqueue(new Callback() {
+      @Override
+      public void onFailure(@NonNull Call call, @NonNull IOException ioException) {
+        ioException.printStackTrace();
+      }
+
+      @Override
+      public void onResponse(@NonNull Call call, @NonNull final Response response)
+          throws IOException {
+        if (!response.isSuccessful()) {
+          throw new IOException("Unexpected code: " + response);
+        } else {
+          contentType = response.header("Content-Type");
+        }
+      }
+    });
+    return contentType;
+  }
 }