ASP.NET Core 5 - How to add MIME type for a custom file extension

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Routing;
using Microsoft.AspNetCore.StaticFiles;

namespace MyContactForm
{
public class Startup
{
public void Configure(IApplicationBuilder app)
{
// using Microsoft.AspNetCore.StaticFiles;
var provider = new FileExtensionContentTypeProvider();

provider.Mappings[".abc"] = "text/html";

app.UseStaticFiles(

new StaticFileOptions
{
ContentTypeProvider = provider
}

);

app.UseRouting();

app.UseEndpoints(endpoints =goesto
{
endpoints.MapGet("/", async context ==goesto
{
await context.Response.WriteAsync("Hello!");
});
});
}
}
}