add javascript preview backend

lib/preview/txt.php

@@ -58,4 +58,14 @@ class PHP extends TXT {
 
 }
 
-\OC\Preview::registerProvider('OC\Preview\PHP');
\ No newline at end of file
+\OC\Preview::registerProvider('OC\Preview\PHP');
+
+class JavaScript extends TXT {
+
+	public function getMimeType() {
+		return '/application\/javascript/';
+	}
+
+}
+
+\OC\Preview::registerProvider('OC\Preview\JavaScript');
\ No newline at end of file