Have you tried this Microsoft tutorial (ASP.NET Core Blazor file uploads | Microsoft Docs
])? If you still can't make it work, please let me know.
Blazor is a new Microsoft solution to replace the old Web Forms & Knockout, both officially retired. Knockout did not live long, about 2-3 years only, dying in its infancy. I don't know if Blazor will follow Knockout due to its poor & non-mainstream design. If you have a choice, making a tiny framework on top of ASP.Net is a better way to go, well, assuming you'll have to maintain the code or manage the project for the next 10 years.
Another popular way is to use Angular/AngularJS, React or Vue as the frontend & raw ASP.Net web services as its backend. Even Microsoft web service frameworks retire too fast.