diff --git a/lib/private/preview.php b/lib/private/preview.php
index 7f219850bb39798414a23283064ae0c59d309bca..ff93f438f7325b2cc05ca6157a88b95c2b94dade 100755
--- a/lib/private/preview.php
+++ b/lib/private/preview.php
@@ -173,6 +173,10 @@ class Preview {
 		return $this;
 	}
 
+	/**
+	 * @brief set mimetype explicitely
+	 * @param string $mimetype
+	 */
 	public function setMimetype($mimetype) {
 		$this->mimetype = $mimetype;
 	}