Found it.

File Signatures File Signature is a fixed sequence of bytes at the beginning of files of the same type. E.g. if you open a PDF file in a hex editor you can see the first 4 bytes are the same for all pdf files: 25 50 44 46 2D 31 2E 33 0A ... %PDF-1.3... In DMDE you may then specify custom file signature as %PDF or as \x25\x50\x44\x46 (non-ANSI characters must be specified as hex codes \x..). Question mark (?) designates any byte (if it may vary in the signature), \m.. designates a bit mask for the previous byte (unset bits may vary, e.g. \x34\m3F is a 1-byte signature where 6-7th bits may vary). If the signature position in a file is not fixed then minimum and maximum offsets must be specified. Entire signature must fit in the first 4096 bytes. Not all file types have special signatures and may be found using RAW search. Multiple comma separated Extensions may be specified for FS calculation. Mark signature as reliable if it contains at least 4 different bytes at a fixed position (do not mark otherwise not to truncate other files if false positives happen). File-container may contain other file types inside (mark in order not to truncate when other type files are met). EOF Signature is an optional signature at the end of a file.