Lets Encrypt ACME Challenge file not accessable from IIS

You just Add a New MIME Type on IIS like this .

and try use the url at your browser . you will see that Now you can Pass the lets-encrypt authentication :)

Also, if you're using a system with lots of custom routing or a framework that interferes with how URLs are handled (e.g. a CMS), ensure that you've told it to ignore /.well-known

We often use Umbraco for public-facing sites and I keep forgetting that I need to add ~/.well-known to the umbracoReservedUrls app setting in the web.config. Hopefully next time I'm stuck, I'll come across this answer...