This repo has a Blazor Web assembly app with an ASP.NET Core API that contains everything you need to know about how to deal with Files from your Blazor Client-Side Apps
YouTube associated playlist https://www.youtube.com/watch?v=NTnul5OE-9A&list=PLFJQnCcZXWjsHh_-fdpNmZJn1LhNm7ck0