Tried to add custom buttons to toolbar.
Then I found it doesn't work as expected. Seems that icons are missing (ok, I didn't define any, but found now way how to do it), but based on html, it generates tooltip as css class :-)
Then I found some strange code in Telerik_UI_for_ASP.NET_AJAX_2018_3_910_Source\Telerik.Web.UI\FileExplorer\RadFileExplorer.cs.
So it seems to be a bug in this method. Css class should not be generated from Tooltip :-)
Please see attachments.