Hi there!
I've read you requirements. You are looking for a API developer who can add authentication to your existing API and add ability to download mp4 files as well.
I believe I can definitely help you out with this.
I am a full stack .NET developer. I have worked on similar projects using similar technologies. I am well versed in developing web solutions in ASP.NET Core, MVC, Web API, DevExpress, Blazor, Angular 6+, Azure, TypeScript, SQL Server, Entity Framework, EF Core and Dapper.
If any of this sounds interesting, please reply to this and we can arrange to discuss your requirements in detail.
Best regards,
Divyansh Bhatia