asp.net core - How to serve static file with no extension

Adding an alternative solution. You have to set ServeUnknownFileTypes to true and after that set the default content type.

        app.UseStaticFiles(new StaticFileOptions
        {
            ServeUnknownFileTypes = true,
            DefaultContentType = "text/plain"
        });

Rather than fighting with static files, I think you'd be better off just creating a controller for it:

using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using System.IO;

namespace MyApp.Controllers {
    [Route("apple-app-site-association")]
    public class AppleController : Controller {
        private IHostingEnvironment _hostingEnvironment;

        public AppleController(IHostingEnvironment environment) {
            _hostingEnvironment = environment;
        }

        [HttpGet]
        public async Task<IActionResult> Index() {
            return Content(
                await File.ReadAllTextAsync(Path.Combine(_hostingEnvironment.WebRootPath, "apple-app-site-association")),
                "text/plain"
            );
        }
    }
}

This assumes your apple-app-site-association file is in your wwwroot folder.

An easier option may be to put a file with a proper extension on the server, and then use URL rewrite as follows.

app.UseRewriter(new RewriteOptions()
    .AddRewrite("(.*)/apple-app-site-association", "$1/apple-app-site-association.json", true));