You can see how you can do this here: https://www.telerik.com/forums/upload-and-display-image-on-form
In OnSelect, you don't have the file because the Upload component does not send it to Blazor - this would be a performance hit for server-side blazor as ultimately the file would be sent over the wire 3 times, all of them over SignalR which is not designed for large payloads. If you want to have a file in OnSelect to modify and use immediately, you would need a FileSelect component that sends the file to the Blazor app, and not to an endpoint. In this article you can Vote for and Follow the implementation of such a Telerik component, and you can also find an example of how to make one yourself: https://feedback.telerik.com/blazor/1460649-fileselect-component.
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