A polymock header, containing many standard signatures at their correct offset, such as Dos' MZ at offset 0, ARJ's 60EA at offset 2...
https://files.mastodon.social/media_attachments/files/111/727/540/523/200/860/original/3d7a1255a2ab029d.png
An extreme example of a weird file construct, applicable to most formats:
a polymock, with fake file formats signatures at their correct offset.
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