The RAW is unclear on what type of ability spellcasting is. MM5 has some monsters that have their spellcasting as an (Ex) ability, while others have argued that it is a natural ability, because it is not otherwise designated as (Ex), (Su) or (Sp). The usual counterargument against being a natural ability is that it is not something that the caster has because of a physical feature.
The text for Spell-Like Abilities strongly suggests that actual spellcasting is different from it, with this text: "an individual creature could have some spell-like abilities and also cast other spells as a sorcerer."
Spells being Supernatural would make some things invalid, such as being able to cast
Invoke Magic in an antimagic field, which is the whole purpose of the spell. This is because the basic spellcasting ability would go away in an antimagic field, rendering the specific text of the spell meaningless, as you would never be able to get that far in casting it.
Thus, by the RAW, it is either (Ex) or natural.