For some reason or another, when I add an invalid file extension the error event isn't happening. Did I overlook something?
$(document).ready(() => {
  IniUploadJSONFile();
});
function IniUploadJSONFile() {
  $("#ImportJSONOrderFile").empty();
  $("#ImportJSONOrderFile").kendoUpload({
    async: {
      //saveUrl: ImportQuote,
      autoUpload: false,
      multiple: false
    },
    validation: {
      allowedExtensions: [".json"]
    },
    error: onError
  });
}
function onError(e) {
  var files = e.files;
  for (var i = 0; i < files.length; i++) {
    alert("Validation failed for " + files[i].name);
    var uid = files[i].uid;
    var entry = $(".k-file[data-uid='" + uid + "']");
    if (entry.length > 0) {
      entry.remove();
    }
  }
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://kendo.cdn.telerik.com/2022.1.412/styles/kendo.common.min.css">
<link rel="stylesheet" href="https://kendo.cdn.telerik.com/2022.1.412/styles/kendo.rtl.min.css">
<link rel="stylesheet" href="https://kendo.cdn.telerik.com/2022.1.412/styles/kendo.default.min.css">
<link rel="stylesheet" href="https://kendo.cdn.telerik.com/2022.1.412/styles/kendo.mobile.all.min.css">
<script src="https://kendo.cdn.telerik.com/2022.1.412/js/jszip.min.js"></script>
<script src="https://kendo.cdn.telerik.com/2022.1.412/js/kendo.all.min.js"></script>
<input id="ImportJSONOrderFile" type="file" />
                        
Here is an example that will only accept
.jsonfile extension. You can run it in the Telerik DOJO.