Changed file_exists to is_file in delete methods
Changed file_exists to is_file in delete methods

In Mediafile::deleteThumbs() and Mediafile::deleteFile(), I had added a file_exists condition in my previous pull request. It's more correct to use is_file, because file_exists returns true also for directories, which clearly we don't want to delete here.
PendalF89/yii2-filemanager,PendalF89/yii2-filemanager