Earlier tonight, I was trying to setup a new file extension for a web application using IIS. The idea was simple enough. Default Website | Home Directory | Configuration | Application Configuration.
I tried clicking the Add button to add an extension, but when I did the OK button was always disabled. No matter what, I could not get it to enable.
This is one of the sillier workarounds I've seen in a while. After selecting the executable, you need to click on the textbox itself and the path will fully expand, enabling the OK button so you can save the mapping.