diff --git a/lib/private/preview.php b/lib/private/preview.php
index 74051fbc2a3dab8d772e8ce81fcff879986aa960..70bfdb6e70ace9554ae8164a1c0c29e3677ff03f 100755
--- a/lib/private/preview.php
+++ b/lib/private/preview.php
@@ -463,7 +463,7 @@ class Preview {
 		if (is_null($this->preview)) {
 			$this->getPreview();
 		}
-		$this->preview->show();
+		$this->preview->show('image/png');
 		return;
 	}