app.UseStaticFiles(new StaticFileOptions
{
  OnPrepareResponse = context =>
  {
    IHeaderDictionary headers = context.Context.Response.Headers;
    string contentType = headers["Content-Type"];
    if (contentType == "application/x-gzip")
    {
      if (context.File.Name.EndsWith("js.gz"))
      {
        contentType = "application/javascript";
      }
      else if (context.File.Name.EndsWith("css.gz"))
      {
        contentType = "text/css";
      }
      headers.Add("Content-Encoding", "gzip");
      headers["Content-Type"] = contentType;
    }
  }
});

var provider = new FileExtensionContentTypeProvider();
string contentType;
if (!provider.TryGetContentType(fileName, out contentType))
{
  contentType = "application/octet-stream";
}

var mimeTypeProvider = new FileExtensionContentTypeProvider();
 
app.UseStaticFiles(new StaticFileOptions
{
  OnPrepareResponse = context =>
  {
    var headers = context.Context.Response.Headers;
    var contentType = headers["Content-Type"];
 
    if (contentType != "application/x-gzip" && !context.File.Name.EndsWith(".gz"))
    {
      return;
    }
 
    var fileNameToTry = context.File.Name.Substring(0, context.File.Name.Length - 3);
 
    if (mimeTypeProvider.TryGetContentType(fileNameToTry, out var mimeType))
    {
      headers.Add("Content-Encoding", "gzip");
      headers["Content-Type"] = mimeType;
    }
  }
});