MAGETWO-95646: Custom file attribute works incorrect

Author: svitja

lib/internal/Magento/Framework/Filesystem/Driver/File.php

@@ -843,6 +843,13 @@ public function fileUnlock($resource)
      */
     public function getAbsolutePath($basePath, $path, $scheme = null)
     {
+        // check if the path given is already an absolute path containing the
+        // basepath. so if the basepath starts at position 0 in the path, we
+        // must not concatinate them again because path is already absolute.
+        if (0 === strpos($path, $basePath)) {
+            return $this->getScheme($scheme) . $path;
+        }
+
         return $this->getScheme($scheme) . $basePath . ltrim($this->fixSeparator($path), '/');
     }