This code might not be perfect but I tested it and it seems to work.

The problem is that the mime type ".stp" isn't recognised by the server as being downloadable. We have to add in code to tell the server to download .stp files.

Add this in startup.cs > Configure method just above app.UseUmbraco():

        var provider = new FileExtensionContentTypeProvider();
        // Set the MIME type for .stp files
        provider.Mappings[".stp"] = "application/octet-stream";

        // Apply middleware only for .stp file requests
        app.UseWhen(
            context => context.Request.Path.HasValue && context.Request.Path.Value.EndsWith(".stp", StringComparison.OrdinalIgnoreCase),
            appBuilder =>
            {
                appBuilder.UseStaticFiles(new StaticFileOptions
                {
                    ContentTypeProvider = provider
                });
            }
        );

Let me know if it works.