How to send files for upload without access to file input tag on the UI

Question

I am trying to upload files using kendoFileManager control in the worker thread as I have some pre-upload check on the files which might take time.

I have following code in my worker -

worker.js

onmessage = function(filesArray) {
var fileData = new FormData();

// if there are multiple files , loop through each files
for (var i = 0; i < message.data.length; i++) {
     fileData.append(message.data[i].name, message.data[i]);
    }
var xhr = new XMLHttpRequest();
xhr.open("POST", "/SelfServiceAI/Upload3/", true);
xhr.setRequestHeader('Accept', 'multipart/form-data');
xhr.withCredentials = true;
xhr.onload = () => { // Call a function when the state changes.
    if (xhr.readyState === XMLHttpRequest.DONE && xhr.status === 200) {
        //postMessage("File successfully uploaded")
        // Request finished. Do processing here.
    }
    else {
        //postMessage("File upload failed!");
    }
}
xhr.send(fileData);
}

This worker is created and invoked on the upload event of the kendo FileManagerControl.

$("#filemanager").kendoFileManager({ 
....
upload : {
          upload : function (e) {
             const worker = new Worker(worker.js);
             worker.postMessage(e.sender.getFiles());
           }
         },
....
});

controller action method -

public JsonResult Upload ()
{
 HttpFileCollectionBase files = Request.Files;
 // Other processing logic
}

For me, the Request.Files is coming up as empty and there is no other property in Request that can be used to get the Files data for upload. I am expecting a way to get the files passed from the UI in the controller action method.