Actually I double checked and triple checked.
The action of my mIRC directly mirrors the "open" action for .htm(l) files. One thing I will give you is that I was modifying .htm and .html at the same time - so I have no way to be CERTAIN it was the .html association. It could be .htm. I will now check.
To be certain that it's not just my client I'd appreciate it if you could run the test you did with the .htm association.
In addition, if you were right about your assumption about .html then how do you explain how the function would work for URL's bearing filenames that end in .asp, .pl, .php, .jsp, etc?
No, I think you misunderstand... mIRC isn't looking up the association for the filename it's opening on the web, it's looking up the .htm(l) association for any URL, regardless of its filename.