Address dynamic class property deprecation warnings

models/Asset.php

@@ -13,6 +13,7 @@ class Asset
 	protected $subdir;
 	protected $filename;
 	protected $title;
+	protected $slug;
 	protected $mimetype;
 	protected $image_width;
 	protected $image_height;
@@ -364,7 +365,7 @@ class Asset
 
 	public function isImage()
 	{
-		return substr($this->mimetype, 0, 5) === 'image';
+		return isset($this->mimetype) && substr($this->mimetype, 0, 5) === 'image';
 	}
 
 	public function getImage()