are those used for wrapper (or container) file formats.

Magic number

One way to incorporate file type metadata is to store a "magic number" inside the file itself. Originally, this term was used for 2-byte identifiers at the start of files, but since any binary sequence can be regarded as a number, any feature of a file format which uniquely distinguishes it can be used for identification. GIF images, for instance, always begin with the ASCII

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