has there been any changes to the editor since the expansion has come?
Yes, the editor has changed considerably with many new natives, data catalogs/classes and editor types (eg cut scene and AI) being added. These changes were part of the StarCraft II core changes that went along with HotS so are included with WoL (they are not exclusive to HotS).
or are they just changing things whenever they feel like it?
That is pretty much what they are doing. The good news is this gives you the full power of the editor even if you do not get the expansions. This approach is the complete opposite of WarCraft III where part of the reason to get the expansion was the improved editor. All the expansion does is add new assets and unlock expansion content (the gripping and fun campaign). Obviously you need the expansion to play maps made using the expansion assets.
also, are there any differences in file name extensions for maps now that there is an expansion?
Of course not as the expansion comes in the form of a mod (dependency). Players who have not purchased the expansion will not be able to use that mod so are prevented from playing the map. Even if you only have WoL, you will still download the entire HotS data, ready for use. To use HotS data you need to attach a HotS key to your account to unlock using the content.
You can even see all HotS campaign maps from inside the editor. If you do not have a HotS licence, they will fail to load fully (unable to load dependency) but things like triggers will still be visible.
are there different file name extensions at all or what's the deal on that?
Both WoL and HotS use the same game engine. All supported extensions are the same. The only difference is what built in assets you have available to you. HotS upgrades many WoL assets to support new engine features, such as physics, that are otherwise only supported on custom user-made models in WoL.
HotS adds many new assets to choose from as well as keeping all assets (except localized assets) from WoL. An example is the addition of over a dozen new Zerg units such as Primal Zerg and Zerg Swarm Mutations. You also get a couple of useful lore units such as a new Alexei Stukov, Kerrigan Ghost form and Narud. There are also a number of boss/hero Zerg units such as the Primal leaders and the Zerg that help Kerrigan. Let us not forget all new melee units such as transformable hellions, mothership core and vipers as well as those units which were unfortunately axed during beta. Finally there is at least 6 new environments to choose from complete with new decorations.
