Currently, before processing the file, the UploadHandler checks if overwriting files is enabled and if the file exists. If needed, it cancels the upload and returns an error to the client. You can override this behavior by overriding the InitializeChunkStorage method. However, I am not sure you will be able to return a message and still continue the upload if needed. You can return a server parameters from the server to the client when the file uploads of fails. Once the file fails, it has to be re-added and re-uploaded.
the Telerik team
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