diff --git a/CHANGELOG.md b/CHANGELOG.md
index 68570da..64ed83b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ## 1.2.0 - (unreleased)
 
 - Refactored MultipartStreamBuilder to clean up and allow injecting data without a filename
+- Dynamically use memory or temp file to buffer the stream content.
 
 ## 1.1.2 - 2020-07-13
 
@@ -24,23 +25,23 @@ No changes from 0.2.0.
 ## 0.2.0 - 2017-02-20
 
 You may do a BC update to version 0.2.0 if you are sure that you are not adding
-multiple resources with the same name to the Builder. 
+multiple resources with the same name to the Builder.
 
 ### Fixed
 
-- Make sure one can add resources with same name without overwrite. 
+- Make sure one can add resources with same name without overwrite.
 
 ## 0.1.6 - 2017-02-16
 
 ### Fixed
 
-- Performance improvements by avoid using `uniqid()`. 
+- Performance improvements by avoid using `uniqid()`.
 
 ## 0.1.5 - 2017-02-14
 
 ### Fixed
 
-- Support for non-readable streams. This fix was needed because flaws in Guzzle, Zend and Slims implementations of PSR-7. 
+- Support for non-readable streams. This fix was needed because flaws in Guzzle, Zend and Slims implementations of PSR-7.
 
 ## 0.1.4 - 2016-12-31
 
@@ -52,7 +53,7 @@ multiple resources with the same name to the Builder.
 
 ### Added
 
-- Added `CustomMimetypeHelper` to allow you to configure custom mimetypes. 
+- Added `CustomMimetypeHelper` to allow you to configure custom mimetypes.
 
 ### Changed
 
@@ -62,13 +63,13 @@ multiple resources with the same name to the Builder.
 
 ### Added
 
-- Support for Outlook msg files. 
+- Support for Outlook msg files.
 
 ## 0.1.1 - 2016-08-10
 
 ### Added
 
-- Support for Apple passbook. 
+- Support for Apple passbook.
 
 ## 0.1.0 - 2016-07-19
 
diff --git a/src/MultipartStreamBuilder.php b/src/MultipartStreamBuilder.php
index a14ab4e..6f94dbe 100644
--- a/src/MultipartStreamBuilder.php
+++ b/src/MultipartStreamBuilder.php
@@ -73,12 +73,10 @@ public function __construct($streamFactory = null)
     }
 
     /**
-     * Add a resource to the Multipart Stream
+     * Add a resource to the Multipart Stream.
      *
-     * @param string|resource|\Psr\Http\Message\StreamInterface $resource
-     *     The filepath, resource or StreamInterface of the data.
-     * @param array $headers
-     *     Additional headers array: ['header-name' => 'header-value'].
+     * @param string|resource|\Psr\Http\Message\StreamInterface $resource the filepath, resource or StreamInterface of the data
+     * @param array                                             $headers  additional headers array: ['header-name' => 'header-value']
      *
      * @return MultipartStreamBuilder
      */
@@ -133,28 +131,40 @@ public function addResource($name, $resource, array $options = [])
      */
     public function build()
     {
-        $streams = '';
+        // Open a temporary read-write stream as buffer.
+        // If the size is less than predefined limit, things will stay in memory.
+        // If the size is more than that, things will be stored in temp file.
+        $buffer = fopen('php://temp', 'r+');
         foreach ($this->data as $data) {
             // Add start and headers
-            $streams .= "--{$this->getBoundary()}\r\n".
-                $this->getHeaders($data['headers'])."\r\n";
+            fwrite($buffer, "--{$this->getBoundary()}\r\n".
+                $this->getHeaders($data['headers'])."\r\n");
 
-            // Convert the stream to string
-            /* @var $contentStream StreamInterface */
+            /** @var $contentStream StreamInterface */
             $contentStream = $data['contents'];
+
+            // Read stream into buffer
             if ($contentStream->isSeekable()) {
-                $streams .= $contentStream->__toString();
+                $contentStream->rewind(); // rewind to beginning.
+            }
+            if ($contentStream->isReadable()) {
+                while (!$contentStream->eof()) {
+                    // Read 1MB chunk into buffer until reached EOF.
+                    fwrite($buffer, $contentStream->read(1048576));
+                }
             } else {
-                $streams .= $contentStream->getContents();
+                fwrite($buffer, $contentStream->__toString());
             }
-
-            $streams .= "\r\n";
+            fwrite($buffer, "\r\n");
         }
 
         // Append end
-        $streams .= "--{$this->getBoundary()}--\r\n";
+        fwrite($buffer, "--{$this->getBoundary()}--\r\n");
+
+        // Rewind to starting position for reading.
+        fseek($buffer, 0);
 
-        return $this->createStream($streams);
+        return $this->createStream($buffer);
     }
 
     /**