Yannick who is a very smart security tester has mentioned to one of his friends that he has found a way of appending data to an existing file using the built in Windows tools and no third party…

Yannick who is a very smart security tester has mentioned to one of his friends that he has found a way of appending data to an existing file using the built in Windows tools and no third party utility. This text appended does not affect the functionality, size, or display within traditional file browsing utilities such as dir or Internet Explorer. What is Yannick making reference to in this case?