How to get from CopySpec to FileCollection?

You have definitely given me some food for thought and due to that I also found an alternative way of achieving this as per what the Bintray people did for their plugin:

https://github.com/bintray/gradle-bintray-plugin/blob/master/src/main/groovy/com/jfrog/bintray/gradle/RecordingCopyTask.groovy