As a fun example of "depends on the container format", one trick people used to use for sharing files on image boards (4chan and the like) was to concatenate a

As a fun example of "depends on the container format", one trick people used to use for sharing files on image boards (4chan and the like) was to concatenate a rar file and jpg file. I don't remember which order it was, but one of the two used a header (read start to end), and the other used a "footer" (read end to start), so you could use it either way depending on what you opened it as. I still have a handful of files which are books in PDF format in a RAR file, and simultaneously the book's cover as a jpg.