diff --git a/CHANGELOG.md b/CHANGELOG.md
index 54f9e223..002a8e11 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,10 @@
+## [3.0.12] - Apr 25, 2021
+* Fixed to apply option to `SendbirdSdk` properly
+* Fixed `sendFileMessage` progress inconsistency
+* Improved stability
+
 ## [3.0.11] - Apr 22, 2021
+* Added `nicknameStartWith` filter in `ApplicationListQuery`
 * Updated pubspec and README
 * Fixed typo
 
diff --git a/README.md b/README.md
index c17d29d7..e7276a02 100644
--- a/README.md
+++ b/README.md
@@ -34,8 +34,8 @@ This section shows you the prerequisites you need to check for using Sendbird Ch
 The minimum requirements for Chat SDK for Flutter are:
 
 - Xcode or Android studio 
-- Dart 2.10.4
-- Flutter 1.22.x or higher
+- Dart 2.10.4 or above
+- Flutter 1.22.0 or higher
 
 ## Getting started
 
diff --git a/example/main.dart b/example/main.dart
index 0d943597..13b29d90 100644
--- a/example/main.dart
+++ b/example/main.dart
@@ -1,13 +1,14 @@
 import 'package:sendbird_sdk/sendbird_sdk.dart';
 
 void main(List<String> arguments) async {
-  // This example uses the Google Books API to search for books about http.
-  // https://developers.google.com/books/docs/overview
-
+  // create sendbird sdk instance with application id
   final sendbird = SendbirdSdk(appId: 'YOUR-APP-ID');
 
   try {
+    // connect sendbird server with user id
     final user = await sendbird.connect('UNIQUE-USER-ID');
+
+    // generate group channel parameters
     final params = GroupChannelParams()
       ..userIds = [user.userId]
       ..operatorUserIds = [user.userId]
@@ -18,6 +19,7 @@ void main(List<String> arguments) async {
     // create group channel
     final channel = await GroupChannel.createChannel(params);
 
+    // send user message to the group channel
     channel.sendUserMessageWithText('Hello World',
         onCompleted: (message, error) {
       // message has been sent successfully
diff --git a/lib/sdk/internal/sendbird_sdk_internal.dart b/lib/sdk/internal/sendbird_sdk_internal.dart
index e7ef069b..66c70ac2 100644
--- a/lib/sdk/internal/sendbird_sdk_internal.dart
+++ b/lib/sdk/internal/sendbird_sdk_internal.dart
@@ -24,9 +24,8 @@ import 'package:sendbird_sdk/utils/async/async_queue.dart';
 import 'package:sendbird_sdk/utils/logger.dart';
 import 'package:sendbird_sdk/utils/parsers.dart';
 
-const sdk_version = '3.0.11';
+const sdk_version = '3.0.12';
 const platform = 'flutter';
-const platform_version = '1.22.5';
 
 /// Internal implementation for main class. Do not directly access this class.
 class SendbirdSdkInternal with WidgetsBindingObserver {
@@ -206,11 +205,11 @@ class SendbirdSdkInternal with WidgetsBindingObserver {
       _onWebSocketError,
     );
 
+    _sessionManager.setAccessToken(accessToken);
+
     apiHost = reconnect ? _state.apiHost : apiHost ?? _getDefaultApiHost();
     wsHost = reconnect ? _state.wsHost : wsHost ?? _getDefaultWsHost();
 
-    _sessionManager.setAccessToken(accessToken);
-
     _state
       ..reconnecting = reconnect
       ..wsHost = wsHost
@@ -224,7 +223,7 @@ class SendbirdSdkInternal with WidgetsBindingObserver {
 
     var params = {
       'p': platform,
-      'pv': platform_version,
+      'pv': Platform.version.split(' ').first,
       'o': Platform.operatingSystem, // os
       'ov': Platform.operatingSystemVersion, // os version
       'sv': sdk_version,
diff --git a/lib/sdk/sendbird_sdk_api.dart b/lib/sdk/sendbird_sdk_api.dart
index 638d82f7..0061d79d 100644
--- a/lib/sdk/sendbird_sdk_api.dart
+++ b/lib/sdk/sendbird_sdk_api.dart
@@ -38,13 +38,12 @@ class SendbirdSdk {
   }) {
     if (_instance != null &&
         (appId == null || appId == _instance._int?.state?.appId)) {
-      _instance.setOptions(options);
       return _instance;
     }
 
     // initialize with different app id, so logout and
     // reinitialize internal obj
-    _instance._int?.logout();
+    _instance = SendbirdSdk._instanceFunction();
     _instance._int = SendbirdSdkInternal(
       appId: appId,
       apiToken: apiToken,
diff --git a/lib/services/network/http_client.dart b/lib/services/network/http_client.dart
index c8b3d3e0..ff11cb12 100644
--- a/lib/services/network/http_client.dart
+++ b/lib/services/network/http_client.dart
@@ -189,12 +189,14 @@ class HttpClient {
 
     body.forEach((key, value) {
       if (value is FileInfo) {
-        request.files.add(http.MultipartFile.fromBytes(
+        final part = http.MultipartFile(
           key,
-          value.file.readAsBytesSync(),
+          value.file.openRead(),
+          value.file.lengthSync(),
           filename: value.name,
           contentType: MediaType.parse(value.mimeType),
-        ));
+        );
+        request.files.add(part);
       } else if (value is List<String>) {
         request.fields[key] = value.join(',');
       } else if (value is List) {
diff --git a/pubspec.yaml b/pubspec.yaml
index 85891ed1..29caf4fd 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,6 +1,6 @@
 name: sendbird_sdk
 description: The Flutter SDK for Sendbird Chat brings modern messenger chat features to your iOS and Android deployments.. 
-version: 3.0.11
+version: 3.0.12
 homepage: https://www.sendbird.com
 repository: https://www.github.com/sendbird/sendbird-sdk-flutter
 documentation: https://sendbird.com/docs/chat/v3/flutter/getting-started/about-chat-sdk