No, I'm afraid that it is not possible to do what you want under the circumstances that you have outlined. Executable files are treated specially (due to their ability to infect a system), so Windows will not let you just run any old file as an executable.
PATHEXT environment variable tells Windows what programs are "executable" and can be "run", but there are limits. Open a command-prompt and append
pathext and try to run one of the
> set pathext=.cos;%pathext%
You will see that either the default "all files" program runs, or if none is configured, that the Open With dialog is presented. Why? Because Windows restricts what can be run for safty reasons. You should be able to register
.cos files as executable (like
.pl, etc.), but even then, it may not be possible since a
.pl file is actually just a document that is opened by a program, not an actual executable.
Another option might be to try to create an association whereby you tell the system that
.cos files are
comfiles. However, the quick test I just attempted did not work and Windows complained that it is not an executable file and returned access denied.