Where does iOS keep your downloaded files ?

[fabiodelorenzo]

If you programmatically download a file, it goes into the Application sandbox:

The location changed for Xcode 6 with iOS 8:

~/Library/Developer/CoreSimulator/Devices/DEVICE_UDID/data/Containers/Data/Application/APPLICATION_ID/Documents

DEVICE_UDID: can be found out in the Device Inspector (Window -> Devices) and Select the simulated device you're running on. The field Identifier contains the device's UDID.
You can also get a list of the simulator UDID's by running the command xcrun simctl list (thanks @JeremyHuddlestonSequoia)

APPLICATION_ID: I didn't find this number somewhere, so I looked into the folders until I found what I wanted. If someone finds out where this number comes from, let me know.

The ID of the application changes every time you start the simulator, but when you keep a finder window open pointing to that folder, you'll always find yourself in the correct folder (no idea how they do that; every other application seems to loose track of the files), which is extremely handy!