Maybe you should have considered user testing the new forum software? Who knows, one of them may have provided you with a useful set of allowed file extensions, such as .dcl. They may have also provided you with a wealth of other useful feedback about the forum software that you could have taken into account. They might even advised you to choose different software altogether, if they had just happened to notice in passing that this one sucks. That way, maybe you could have avoided inflicting this regurgitated dog's breakfast on your customers.
Given the common use of colons in AutoLisp functions, such as the command-name argument in (defun), and the fact that colons can also be used in other ways such as in variable names, it is really stupid that certain combinations of colons with other characters trigger dumb little faces. What do we need those for, on a forum like this? If people want to use emoticons in their message content outside code portions, I imagine they will settle for the old text-characters-only versions. 8^(
Yes, I have my "Emoticon type" set to "None," so when I'm Signed In, I don't see them. But even I sometimes check into the website and look at some things without necessarily signing in, and am therefore victimized by them. And anyone who isn't registered is out of luck, with no way to know what should go in the code there instead, unless they happen to know which emoticon represents which character combination [I just posted a Translator, to tide us over, and to soon become obsolete, I hope].
It wouldn't be so bad if, when content that includes them is copied and pasted into a plain-text editor like Notepad, the colon-and-character combinations were restored. But they're not -- they're just missing.