As stated in the Task Streams
article, promotion occurs for any change in the task stream where the file action is not a copy
This includes files copied from other parts of the server, not just files copied from the parent stream.
So if you integrate files from another location to a task stream, you should then open the modified files for edit and submit the change for the files to be promoted. Once the files are promoted they will appear in the list of files copied to the parent.
An exception to this rule relates to files copied from one location in the task stream to another location in the task stream, these are promoted automatically. (as of 2013.1/628821)