diff --git a/Slack/Client.php b/Slack/Client.php
index 414c877..5f3fe62 100644
--- a/Slack/Client.php
+++ b/Slack/Client.php
@@ -27,11 +27,12 @@ public function __construct(Connection $connection)
 
     /**
      * @param string $action
-     * @param array $parameter
+     * @param array  $parameter
+     * @param bool   $multipart Whether to use multipart/form-data or the default application/x-www-form-urlencoded
      *
      * @return Response|bool
      */
-    public function send($action, array $parameter = [])
+    public function send($action, array $parameter = [], $multipart = false)
     {
         if (!$this->connection->isValid()) {
             return false;
@@ -51,7 +52,8 @@ public function send($action, array $parameter = [])
         do {
             $response = $this->executeRequest(
                 $url,
-                $this->connection->getHttpMethod() === Request::METHOD_POST ? $parsedRequestParams : null
+                $this->connection->getHttpMethod() === Request::METHOD_POST ? $parsedRequestParams : null,
+                $multipart
             );
             $response = Response::parseGuzzleResponse($response, $action);
 
@@ -89,18 +91,36 @@ protected function buildUri(Actions\ActionsInterface $action, array $requestPara
 
     /**
      * @param Uri $uri
-     * @param array $params form post values
+     * @param array $params    form post values
+     * @param bool  $multipart Whether to use multipart/form-data or the default application/x-www-form-urlencoded
      *
      * @return \GuzzleHttp\Psr7\Response
      */
-    protected function executeRequest(Uri $uri, array $params = null)
+    protected function executeRequest(Uri $uri, array $params = null, $multipart = false)
     {
         $guzzle = new GuzzleClient(['verify' => $this->connection->getVerifySsl()]);
 
+        if (null !== $params) {
+            if ($multipart) {
+                $multipartParams = [];
+                foreach ($params as $paramName => $paramValue) {
+                    $multipartParams[] = [
+                        'name'     => $paramName,
+                        'contents' => $paramValue,
+                    ];
+                }
+                $postParams = ['multipart' => $multipartParams];
+            } else {
+                $postParams = ['form_params' => $params];
+            }
+        } else {
+            $postParams = [];
+        }
+
         return $guzzle->request(
             $this->connection->getHttpMethod(),
             $uri,
-            null !== $params ? ['form_params' => $params] : []
+            $postParams
         );
     }
 }
diff --git a/Slack/Client/Actions/FilesUpload.php b/Slack/Client/Actions/FilesUpload.php
index 661d795..0b83f2b 100644
--- a/Slack/Client/Actions/FilesUpload.php
+++ b/Slack/Client/Actions/FilesUpload.php
@@ -14,7 +14,7 @@ class FilesUpload implements ActionsInterface
      * @var array
      */
     protected $parameter = [
-        'content'         => null,
+        'file'            => null,
         'filetype'        => null,
         'filename'        => null,
         'title'           => null,
diff --git a/Slack/Messaging.php b/Slack/Messaging.php
index 291348e..24ad269 100644
--- a/Slack/Messaging.php
+++ b/Slack/Messaging.php
@@ -72,16 +72,16 @@ public function message($channel, $message, $identity, array $attachments = [],
         );
     }
 
-
     /**
      * @param string|array $channel # String is DEPRECATED scince v1.3 - Deliver Array
      * @param string       $title
      * @param string       $file
      * @param string|null  $comment
+     * @param string|null  $threadTs
      *
      * @return Client\Response|false
      */
-    public function upload($channel, $title, $file, $comment = null)
+    public function upload($channel, $title, $file, $comment = null, $threadTs = null)
     {
         if (!file_exists($file)) {
             return false;
@@ -96,13 +96,14 @@ public function upload($channel, $title, $file, $comment = null)
         $params['title'] = $title;
         $params['initial_comment'] = $comment;
         $params['channels'] = implode(',', $channel);
-        $params['content'] = file_get_contents($file);
-        $params['fileType'] = mime_content_type($file);
         $params['filename'] = basename($file);
+        $params['file'] = fopen($file, 'r');
+        $params['thread_ts'] = $threadTs;
 
         return $this->client->send(
             Actions::ACTION_FILES_UPLOAD,
-            $params
+            $params,
+            true
         );
     }
 }