Here's the full run down:
Every single attack from a creep, with the exception of boss spells, are "physical." Any projectile attacks are considered "ranged" (and "physical-ranged" if they are also physical) whereas any attacks from enemies that don't produce a visible projectile are either "melee" (mad ghouls, cultists) or "area" (splitter burst damage).
For defenders, the only ones who have magical attacks are healers and ice mages. Healer light is considered magical, as is every ice mage attack (since they are mages). We thought about making the dragon's fire be magic for a while, but decided in the end to treat it as "physical." So, fire breath and fire ball are "physical" and "ranged", though fire ball is technically not "physical-ranged" specifically so that NG+ worms can't dodge it (that nuance is admittedly kind of a hack).
In the end, magic resistance is mostly only of use against bosses. I'm pretty sure there's not a single normal enemy in the game that uses magic. This does mean that magic resistance is generally a lot less useful than physical resistance, but (not having played NG+ all the way through myself) I think it can be a big help against bosses, and a life-saver against an ill-timed "decimate" attack from Zelemir or Eztli.