How to differentiate attached file depends on Revision

Hi, I have a document, which is in revision B, Now I want to differentiate files which are attached when a document is in Revision A and in Revision B. I try to visible created_on property which is on "Document File" relationship ItemType so that I can differentiate them with the help of date but when I attach a new file to the document, the created_on value of all documents get the current date and time.   Regards, Maddy.