Allow setting mimetype via mimetype param in buildAset

mjwshared.php

@@ -179,5 +179,9 @@ function mjwshared_symfony_civicrm_buildAsset($event, $hook) {
     if (empty($event->content) && !empty($event->params['path'])) {
       $event->content = file_get_contents($event->params['path']);
     }
+    if (empty($event->mimeType) && !empty($event->params['mimetype'])) {
+      $event->mimeType = $event->params['mimetype'];
+    }
   }
+
 }