With regard to the widget's functionality when it comes to showing custom messages, it allows you to use a template. In it you can nest arbitrary Html and show/hide some of the elements conditionally.
Here's a dojo example, that shows how you can have a template and still display validation messages by showing/hiding the span with class "validation-message", depending on the extension of the selected file: https://dojo.telerik.com/aGiLudOl
Consider using a similar approach in your scenario - show/hide the element that contains the message with jQuery, when the error event fires.
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