diff --git a/src/ApacheMimetypeHelper.php b/src/ApacheMimetypeHelper.php
new file mode 100644
index 0000000..fdb19a8
--- /dev/null
+++ b/src/ApacheMimetypeHelper.php
@@ -0,0 +1,138 @@
+<?php
+
+namespace Http\Message\MultipartStream;
+
+/**
+ * This class helps to find the proper mime types. The source of this file is taken
+ * from Guzzle.
+ *
+ * @author Michael Dowling and contributors to guzzlehttp/psr7
+ * @author Tobias Nyholm <tobias.nyholm@gmail.com>
+ */
+class ApacheMimetypeHelper implements MimetypeHelper
+{
+    /**
+     * {@inheritdoc}
+     *
+     * @link http://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x/conf/mime.types
+     */
+    public function getMimetypeFromFilename($filename)
+    {
+        return $this->getMimetypeFromExtension(pathinfo($filename, PATHINFO_EXTENSION));
+    }
+
+    /**
+     * {@inheritdoc}
+     *
+     * @link http://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x/conf/mime.types
+     */
+    public function getMimetypeFromExtension($extension)
+    {
+        static $mimetypes = [
+            '7z' => 'application/x-7z-compressed',
+            'aac' => 'audio/x-aac',
+            'ai' => 'application/postscript',
+            'aif' => 'audio/x-aiff',
+            'asc' => 'text/plain',
+            'asf' => 'video/x-ms-asf',
+            'atom' => 'application/atom+xml',
+            'avi' => 'video/x-msvideo',
+            'bmp' => 'image/bmp',
+            'bz2' => 'application/x-bzip2',
+            'cer' => 'application/pkix-cert',
+            'crl' => 'application/pkix-crl',
+            'crt' => 'application/x-x509-ca-cert',
+            'css' => 'text/css',
+            'csv' => 'text/csv',
+            'cu' => 'application/cu-seeme',
+            'deb' => 'application/x-debian-package',
+            'doc' => 'application/msword',
+            'docx' => 'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
+            'dvi' => 'application/x-dvi',
+            'eot' => 'application/vnd.ms-fontobject',
+            'eps' => 'application/postscript',
+            'epub' => 'application/epub+zip',
+            'etx' => 'text/x-setext',
+            'flac' => 'audio/flac',
+            'flv' => 'video/x-flv',
+            'gif' => 'image/gif',
+            'gz' => 'application/gzip',
+            'htm' => 'text/html',
+            'html' => 'text/html',
+            'ico' => 'image/x-icon',
+            'ics' => 'text/calendar',
+            'ini' => 'text/plain',
+            'iso' => 'application/x-iso9660-image',
+            'jar' => 'application/java-archive',
+            'jpe' => 'image/jpeg',
+            'jpeg' => 'image/jpeg',
+            'jpg' => 'image/jpeg',
+            'js' => 'text/javascript',
+            'json' => 'application/json',
+            'latex' => 'application/x-latex',
+            'log' => 'text/plain',
+            'm4a' => 'audio/mp4',
+            'm4v' => 'video/mp4',
+            'mid' => 'audio/midi',
+            'midi' => 'audio/midi',
+            'mov' => 'video/quicktime',
+            'mp3' => 'audio/mpeg',
+            'mp4' => 'video/mp4',
+            'mp4a' => 'audio/mp4',
+            'mp4v' => 'video/mp4',
+            'mpe' => 'video/mpeg',
+            'mpeg' => 'video/mpeg',
+            'mpg' => 'video/mpeg',
+            'mpg4' => 'video/mp4',
+            'oga' => 'audio/ogg',
+            'ogg' => 'audio/ogg',
+            'ogv' => 'video/ogg',
+            'ogx' => 'application/ogg',
+            'pbm' => 'image/x-portable-bitmap',
+            'pdf' => 'application/pdf',
+            'pgm' => 'image/x-portable-graymap',
+            'png' => 'image/png',
+            'pnm' => 'image/x-portable-anymap',
+            'ppm' => 'image/x-portable-pixmap',
+            'ppt' => 'application/vnd.ms-powerpoint',
+            'pptx' => 'application/vnd.openxmlformats-officedocument.presentationml.presentation',
+            'ps' => 'application/postscript',
+            'qt' => 'video/quicktime',
+            'rar' => 'application/x-rar-compressed',
+            'ras' => 'image/x-cmu-raster',
+            'rss' => 'application/rss+xml',
+            'rtf' => 'application/rtf',
+            'sgm' => 'text/sgml',
+            'sgml' => 'text/sgml',
+            'svg' => 'image/svg+xml',
+            'swf' => 'application/x-shockwave-flash',
+            'tar' => 'application/x-tar',
+            'tif' => 'image/tiff',
+            'tiff' => 'image/tiff',
+            'torrent' => 'application/x-bittorrent',
+            'ttf' => 'application/x-font-ttf',
+            'txt' => 'text/plain',
+            'wav' => 'audio/x-wav',
+            'webm' => 'video/webm',
+            'wma' => 'audio/x-ms-wma',
+            'wmv' => 'video/x-ms-wmv',
+            'woff' => 'application/x-font-woff',
+            'wsdl' => 'application/wsdl+xml',
+            'xbm' => 'image/x-xbitmap',
+            'xls' => 'application/vnd.ms-excel',
+            'xlsx' => 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
+            'xml' => 'application/xml',
+            'xpm' => 'image/x-xpixmap',
+            'xwd' => 'image/x-xwindowdump',
+            'yaml' => 'text/yaml',
+            'yml' => 'text/yaml',
+            'zip' => 'application/zip',
+        ];
+
+        $extension = strtolower($extension);
+
+        return isset($mimetypes[$extension])
+            ? $mimetypes[$extension]
+            : null;
+    }
+}
diff --git a/src/MimetypeHelper.php b/src/MimetypeHelper.php
index d16b847..fa7cf18 100644
--- a/src/MimetypeHelper.php
+++ b/src/MimetypeHelper.php
@@ -3,13 +3,9 @@
 namespace Http\Message\MultipartStream;
 
 /**
- * This class helps to find the proper mime types. The source of this file is taken
- * from Guzzle.
- *
- * @author Michael Dowling and contributors to guzzlehttp/psr7
  * @author Tobias Nyholm <tobias.nyholm@gmail.com>
  */
-class MimetypeHelper
+interface MimetypeHelper
 {
     /**
      * Determines the mimetype of a file by looking at its extension.
@@ -18,127 +14,14 @@ class MimetypeHelper
      *
      * @return null|string
      */
-    public static function getMimetypeFromFilename($filename)
-    {
-        return static::getMimetypeFromExtension(pathinfo($filename, PATHINFO_EXTENSION));
-    }
+    public function getMimetypeFromFilename($filename);
 
     /**
      * Maps a file extensions to a mimetype.
      *
-     * @param string $extension The file extension.
+     * @param string $extension The file extension
      *
      * @return string|null
-     *
-     * @link http://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x/conf/mime.types
      */
-    public static function getMimetypeFromExtension($extension)
-    {
-        static $mimetypes = [
-            '7z' => 'application/x-7z-compressed',
-            'aac' => 'audio/x-aac',
-            'ai' => 'application/postscript',
-            'aif' => 'audio/x-aiff',
-            'asc' => 'text/plain',
-            'asf' => 'video/x-ms-asf',
-            'atom' => 'application/atom+xml',
-            'avi' => 'video/x-msvideo',
-            'bmp' => 'image/bmp',
-            'bz2' => 'application/x-bzip2',
-            'cer' => 'application/pkix-cert',
-            'crl' => 'application/pkix-crl',
-            'crt' => 'application/x-x509-ca-cert',
-            'css' => 'text/css',
-            'csv' => 'text/csv',
-            'cu' => 'application/cu-seeme',
-            'deb' => 'application/x-debian-package',
-            'doc' => 'application/msword',
-            'docx' => 'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
-            'dvi' => 'application/x-dvi',
-            'eot' => 'application/vnd.ms-fontobject',
-            'eps' => 'application/postscript',
-            'epub' => 'application/epub+zip',
-            'etx' => 'text/x-setext',
-            'flac' => 'audio/flac',
-            'flv' => 'video/x-flv',
-            'gif' => 'image/gif',
-            'gz' => 'application/gzip',
-            'htm' => 'text/html',
-            'html' => 'text/html',
-            'ico' => 'image/x-icon',
-            'ics' => 'text/calendar',
-            'ini' => 'text/plain',
-            'iso' => 'application/x-iso9660-image',
-            'jar' => 'application/java-archive',
-            'jpe' => 'image/jpeg',
-            'jpeg' => 'image/jpeg',
-            'jpg' => 'image/jpeg',
-            'js' => 'text/javascript',
-            'json' => 'application/json',
-            'latex' => 'application/x-latex',
-            'log' => 'text/plain',
-            'm4a' => 'audio/mp4',
-            'm4v' => 'video/mp4',
-            'mid' => 'audio/midi',
-            'midi' => 'audio/midi',
-            'mov' => 'video/quicktime',
-            'mp3' => 'audio/mpeg',
-            'mp4' => 'video/mp4',
-            'mp4a' => 'audio/mp4',
-            'mp4v' => 'video/mp4',
-            'mpe' => 'video/mpeg',
-            'mpeg' => 'video/mpeg',
-            'mpg' => 'video/mpeg',
-            'mpg4' => 'video/mp4',
-            'oga' => 'audio/ogg',
-            'ogg' => 'audio/ogg',
-            'ogv' => 'video/ogg',
-            'ogx' => 'application/ogg',
-            'pbm' => 'image/x-portable-bitmap',
-            'pdf' => 'application/pdf',
-            'pgm' => 'image/x-portable-graymap',
-            'png' => 'image/png',
-            'pnm' => 'image/x-portable-anymap',
-            'ppm' => 'image/x-portable-pixmap',
-            'ppt' => 'application/vnd.ms-powerpoint',
-            'pptx' => 'application/vnd.openxmlformats-officedocument.presentationml.presentation',
-            'ps' => 'application/postscript',
-            'qt' => 'video/quicktime',
-            'rar' => 'application/x-rar-compressed',
-            'ras' => 'image/x-cmu-raster',
-            'rss' => 'application/rss+xml',
-            'rtf' => 'application/rtf',
-            'sgm' => 'text/sgml',
-            'sgml' => 'text/sgml',
-            'svg' => 'image/svg+xml',
-            'swf' => 'application/x-shockwave-flash',
-            'tar' => 'application/x-tar',
-            'tif' => 'image/tiff',
-            'tiff' => 'image/tiff',
-            'torrent' => 'application/x-bittorrent',
-            'ttf' => 'application/x-font-ttf',
-            'txt' => 'text/plain',
-            'wav' => 'audio/x-wav',
-            'webm' => 'video/webm',
-            'wma' => 'audio/x-ms-wma',
-            'wmv' => 'video/x-ms-wmv',
-            'woff' => 'application/x-font-woff',
-            'wsdl' => 'application/wsdl+xml',
-            'xbm' => 'image/x-xbitmap',
-            'xls' => 'application/vnd.ms-excel',
-            'xlsx' => 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
-            'xml' => 'application/xml',
-            'xpm' => 'image/x-xpixmap',
-            'xwd' => 'image/x-xwindowdump',
-            'yaml' => 'text/yaml',
-            'yml' => 'text/yaml',
-            'zip' => 'application/zip',
-        ];
-
-        $extension = strtolower($extension);
-
-        return isset($mimetypes[$extension])
-            ? $mimetypes[$extension]
-            : null;
-    }
+    public function getMimetypeFromExtension($extension);
 }
diff --git a/src/MultipartStreamBuilder.php b/src/MultipartStreamBuilder.php
index 2fe302f..50c858b 100644
--- a/src/MultipartStreamBuilder.php
+++ b/src/MultipartStreamBuilder.php
@@ -20,6 +20,11 @@ class MultipartStreamBuilder
      */
     private $streamFactory;
 
+    /**
+     * @var MimetypeHelper
+     */
+    private $mimetypeHelper;
+
     /**
      * @var string
      */
@@ -49,7 +54,7 @@ public function __construct(StreamFactory $streamFactory = null)
      *     @var array $headers additional headers ['header-name' => 'header-value']
      *     @var string $filename
      * }
-     * 
+     *
      * @return MultipartStreamBuilder
      */
     public function addResource($name, $resource, array $options = [])
@@ -130,7 +135,7 @@ private function prepareHeaders($name, StreamInterface $stream, $filename, array
 
         // Set a default Content-Type if one was not provided
         if (!$this->hasHeader($headers, 'content-type') && $hasFilename) {
-            if ($type = MimetypeHelper::getMimetypeFromFilename($filename)) {
+            if ($type = $this->getMimetypeHelper()->getMimetypeFromFilename($filename)) {
                 $headers['Content-Type'] = $type;
             }
         }
@@ -198,4 +203,30 @@ public function setBoundary($boundary)
 
         return $this;
     }
+
+    /**
+     * @return MimetypeHelper
+     */
+    private function getMimetypeHelper()
+    {
+        if ($this->mimetypeHelper === null) {
+            $this->mimetypeHelper = new ApacheMimetypeHelper();
+        }
+
+        return $this->mimetypeHelper;
+    }
+
+    /**
+     * If you have custom file extension you may overwrite the default MimetypeHelper with your own.
+     *
+     * @param MimetypeHelper $mimetypeHelper
+     *
+     * @return MultipartStreamBuilder
+     */
+    public function setMimetypeHelper(MimetypeHelper $mimetypeHelper)
+    {
+        $this->mimetypeHelper = $mimetypeHelper;
+
+        return $this;
+    }
 }