Fix for issue magento#716. Wrong mimetype returned by magento image library

Author: flavien.chantelot

lib/internal/Magento/Framework/Image/Adapter/AbstractAdapter.php

@@ -291,12 +291,12 @@ public function __construct(\Magento\Framework\Filesystem $filesystem, array $da
      * Assign image width, height, fileType and fileMimeType to object properties
      * using getimagesize function
      *
-     * @return int|null
+     * @return string
      */
     public function getMimeType()
     {
-        if ($this->_fileType) {
-            return $this->_fileType;
+        if ($this->_fileMimeType) {
+            return $this->_fileMimeType;
         } else {
             list($this->_imageSrcWidth, $this->_imageSrcHeight, $this->_fileType, ) = getimagesize($this->_fileName);
             $this->_fileMimeType = image_type_to_mime_type($this->_fileType);