The web.config no extension mime problem Lets Encrypt on IIS/Windows

 Tip when using LetsEncrypt in Windows - Feature Requests - Let's Encrypt Community Support

In a paragraph, just use the web.config file to set the mime type, eg... 

After trying to figure out why my Lets Encrypt failed to generate in Windows, and discovering that it's the no extension mime type problem, I wondered whether certbot could fill in the missing gap.

All that is needed is the following web.config file to be placed in the same directory as the challenge

<?xml version="1.0" encoding="UTF-8"?>
 <configuration>
     <system.webServer>
         <staticContent>
             <mimeMap fileExtension="." mimeType="text/xml" />
         </staticContent>
     </system.webServer>
 </configuration>

Because there's no mime type for files without an extension on IIS, IIS sends back a 404 when verification happens. The web.config file above sets the mime type.

After I created the .well-known folder and the acme-challenge folder, and inserted the web.config there, it all worked fine.

Once You've got the pems, do something like... in openssl 

openssl pkcs12 -export -out websitename.pfx -inkey privkey1.pem -in fullchain1.pem 

See also, DigiCert Certificate Utility for Windows | DigiCert.com

See also, Install Linux Subsystem on Windows Server | Microsoft Docs