My question is regarding a hidden file inside another file. However, it is not a specific problem I have about programming (that discards StackOverflow, I think) but instead a "How does it work?" question.

My question is:

I know I can build a .rar file with regular method or programs, and append it to an image by using a command such as cat img.jpg myfile.rar > img.jpg and when opening the image, the image will be displayed as before. But I can also open the image with WinRAR or a software alike (like unrar e img.jpg). How is possible that the rar file is recognized inside the jpg image, instead of rejected as not being an appropriate file format?

But, seriously, I cannot figure in which site is it appropriate to ask it (perhaps the question even exists in such site and I am not aware of it, because I don't know the appropriate site from start).

Where can I ask this question?

  • The closest I can think off would be programmers.se but I doubt if they take this question as they have very strict rules and non-regulars most often get it wrong, including me. You could check in their chat room first to see what they think.
    – rene
    May 26, 2016 at 19:41
  • 1
    @rene at Programmers it will likely fail not by topicality but by scope (Why is research important?). It's "explain me jpg format specification" in disguise
    – gnat
    May 26, 2016 at 19:46

1 Answer 1


This looks like a general computer question, not specific to any operating system, not security-related, not programming-related. I suggest Super User.

  • Will check the scope May 26, 2016 at 19:50
  • 1
    Actually, check out this question before you ask. May 26, 2016 at 19:53
  • I don't know who downvoted. I am upvoting since it seems to be well-received in such site. If it is actually well received, you will have my acceptance (my question is slightly different to the one you linked). May 26, 2016 at 19:57

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