Does fire resist granted by an Extraordinary ability stack with magic granted from a magical source? Eg. a sorcerer's red draconic bloodline fire resist 5 + armor of resist fire 5 for a total of 10 fire resist. Ex is mundane, while the armor is magical.
No, different sources of Energy Resistance do not stack with one another, they overlap.
In this case, it should be reasonably clear from the Sorcerer bloodline granting the Energy Resistance special ability, which does explicitly not stack with Energy Resistance granted by spells, and the Energy Resistance granted by the armor enhancement is mentioned as functioning "similar to the resist energy spell".
More generally, it is because stacking rules for this special ability are not defined, and thus defaults to the general rule of not stacking. In other words, bonus types are not defined for Energy Resistance. One being magical and the other non-magical does not affect matters.