I think the answer is that the file would be about as secure as if it was stored anywhere else that someone could potentially access it (unless the IPFS nodes hosting the file are in a private swarm that you are in total control of). So pretty good :), but not invulnerable to a motivated attacker using methods like rubber-hose or black-bag cryptanalysis.
So it's probably secure for now. It's probably a good idea to assume that RSA or IDEA will be broken at some point in the future (years or decades from now?) so anyone with the encrypted version of the file might be able to see what's in it.
In order for the public IPFS swarm to work, your node advertises to the swarm hashes that you can provide. Theoretically someone could be listening to all of this traffic and download everything it hears about.