A few tidies as a result of the Meterpreter gemification code

As noted in https://github.com/rapid7/metasploit-framework/pull/3466 there were a few small formatting/style issues that needed adjusting. They are:

* Use of `SortedSet` instead of arrays for generation of extension lists/tabs.
* Indentation of `when` statements in `case` blocks.
* Use of wildcards in the `.gitignore` for the ignored extensions.

There might be other things which will be adjusted down the track but this should be good to go short term.