IIS 7.5 problems LetsEncrypt HTTP validation due to not serving extension-less files

I am struggling with generating LetsEncrypt SAN SSL for SBS 2011 for few days. All is going fine, until ACME CHALLENGE verification. I cannot use DNS verification, because DNS is at ISP and it takes days for any change to get live. So only HTTP validation can be used.

Where IIS stucks?

Simply when it tries to server extension-less ACME VALIDATION file, IIS returns 404 ERROR. File is there, Acme client generates it just fine in proper folder, but it does not show up via web browser, just 404 error due to MIME type. When testing with test.html file in same folder it gets displayed properly, no problem.

I've already tried:

• Adding MIME type text/plain for "." and ".*" extensions, but no go
• Moved StaticFile mappings above ExtensionLessUrlHandlers, but still no go
• Edited applicationhost.config file and set to Allow: <section name="handlers" overrideModeDefault="Allow" />
• Restarted IIS and whole server, still at no avail
• Used different LE clients, but all of them use IIS and stuck at the same point
• Solution from here does NOT work: IIS: How to serve a file without extension?

When I try localy, I always get this 404 Error in browser:

HTTP Error 404.0 - Not Found
The resource you are looking for has been removed, had its name changed, or is temporarily unavailable.

Module  IIS Web Core
Notification    MapRequestHandler
Handler StaticFile
Error Code  0x80070002

Any more idea?

Sorry, folks! It was my bad, not being carefull enough when passing details to you. The solution to add "." as MIME Type "text/plain" is the only thing needed in my OP case.

What was wrong in my case was the "autodiscover" sub-domain, which I still do not know, where it's being served from, but definitelly it is NOT from "Autodiscover" application under Default Web Site. As of now, when I browse "autodiscover.domain.com..." link I still get cached test.html content, but I've deleted all test.html files which I planted there. Ok, but, that's not the subject here.

BTW...LE test failed on my Firewall on Country Blocking rules. Oh, my...

Thank you for participation.