The Feat *Dragonfire Strike* has the following wording:

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> When you gain extra damage from a sneak attack, sudden strike, or skirmish, you can choose for the extra damage to be fire damage.
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This makes it clear that the attack gains fire descriptor and it's resulting immunities. What is unclear is what it does in relation to Sneak attack immunities, like that of Undead or Constructs. 

I have two thoughts on this:

1. This doesn't change it from being a sneak attack, therefore creature immune to sneak attack are immune to Dragonfire Strike's fire damage.
2. This changes the attack from a sneak attack to a fire based attack thus bypassing Sneak Attack Immunity, but now susceptible to Fire Immunity instead.

Which is correct?