External Device Connection With iOS Devices

For a project which needed cable connection and file transfer with iOS Devices, we need to accomplish OTG cable or a different cable connection. Our devices are kind of Arduino and we trying to transfer data like mp3 files to from iOS devices, but there is not a source on the internet about file transfer with cable for an iOS device. We just need to figure it out with how we can do these with using SWIFT. If you have a knowledge about it please contact us.