Dependency Injection code for InMemoryDatabase in ASP.Net Core

I am working on a POC and need some data. Decided to use the following as a template:

https://stormpath.com/blog/tutorial-entity-framework-core-in-memory-database-asp-net-core

Followed the example. Except at opt.UseInMemoerydatabase("Files"). without the name it shows error. This is my code. It throws

System.InvalidOperationException
  HResult=0x80131509
  Message=Cannot resolve scoped service 'DataAPI.Models.FileContext' from root provider.

at

var context = app.ApplicationServices.GetService<FileContext>();

Any one can suggest what am I doing wrong

namespace DataAPI
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            // Register the Swagger generator, defining one or more Swagger documents
            services.AddSwaggerGen(c =>
            {
                c.SwaggerDoc("v1", new OpenApiInfo { Title = "My DataAPI", Version = "v1" });
            });
            services.AddDbContext<FileContext>(opt =>
                {
                    opt.UseInMemoryDatabase("Files");
                });
            services.AddControllers();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            app.UseHttpsRedirection();
            app.UseRouting();
            app.UseAuthorization();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller=Home}/{action=Index}/{id?}");
            });
            app.UseSwaggerUI(c =>
            {
                c.SwaggerEndpoint("/swagger/v1/swagger.json", "My DataAPI V1");
            });

            var context = app.ApplicationServices.GetService<FileContext>();
            AddTestData(context);
        }

Here is the context class. File is a simple class with a few fields like id, name

namespace DataAPI.Models
{
    public class FileContext: DbContext
    {
        public FileContext(DbContextOptions<FileContext> options)
            : base(options)
        {
        }

        public DbSet<File> Files { get; set; }

    }
}