From what I understand, you want to attach the same file that is attached to a different record.
In other words, when a file is attached to a record in App A, the same file is attached to a record in App B. Correct me if I'm wrong.
The flow should be something like the following:
1) Obtain a file key of the attached file from an app using the GET method
2) Download the file with the obtained file key from 1)
3) Get a new file key of the downloaded file
4) Using the file key that you obtained in 3) and the POST method, upload the file to a different app
Examples on how to download/upload a file using a file key are described in
the following pages so please use those pages for your reference:
Kintone Developer Program - Download File
Kintone Developer Program - Upload File
I hope this helps!