Liberty BASIC doesn't map the BAS file extension to itself when it is installed. This is because I didn't want to be so presumptuous as to steal a common file type away from another version of BASIC that might be installed.
So, because there is a bug in Windows which makes it really hard to remap a file extension and there have been discussions about this in the Liberty BASIC forum at conforums.com one of our member Chris Iverson (thanks Chris!) has contributed some Liberty BASIC code to solve this problem.
Click to see thread in the forum