How to use hexadecimal code as a magic value in mime-type file

I know to add a mimetype I must use a code like that:

<mime-type type="audio/x-gtp">
    <comment>GuitarPro file</comment>
    <glob pattern="*.gp5"/>
</mime-type>

I know to add a mimetype whose file extension is used for other mimetype I can use magic like that:

<mime-type type="audio/x-gtp">
    <magic priority="50">
        <match value="BCFZ" type="string" offset="0" />
    </magic>
    <glob pattern="*.gpx"/>
</mime-type>

This, taking into account that the first 4 characters of the file are printable. I obtained this pattern using xxd. I show you the first four lines of xxd output:

xxd '/home/cactus/Descargas/Billie Eilish - Everything I Wanted.gpx'  | head -n 4 
00000000: 4243 465a 0420 0500 6848 68cd 4c00 0180  BCFZ. ..hHh.L...
00000010: 0000 4ed7 fdff feff ffff e76d 156a 52d8  ..N........m.jR.
00000020: 516e 142e 0141 7201 00e4 0201 c804 0390  Qn...Ar.........
00000030: 0807 2010 0e40 201c 8040 3900 8072 0100  .. ..@ ..@9..r..

So far, everything works fine.

The problem with this other filetype .gp, whose first characters are not imprimible, I show you the output of the first four xdd lines:

xxd /home/cactus/Descargas/arpeggio.gp   | head -n 4                                                                                                                                                      
00000000: 504b 0304 1400 0000 0000 0000 0000 0000  PK..............
00000010: 0000 0000 0000 0000 0000 0800 0000 436f  ..............Co
00000020: 6e74 656e 742f 504b 0304 1400 0800 0800  ntent/PK........
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................

So I don't know how use magic with that type of files, I think the best way would be using hexadecimal code in value, but I don't know how use it.

Thanks for your time.

I was able to solve the problem.

In that page https://specifications.freedesktop.org/shared-mime-info-spec/latest/ar01s02.html#idm45785599133248 I found the following definition:

value: The value to compare the file contents with, in the format indicated by the type attribute. The string type supports the C character escapes (\0, \t, \n, \r, \xAB for hex, \777 for octal).

So that indicates what I needed, so you can use hexadecimal values in the string type using the C character escapes.

Remembering the hexadecimal output of my file:

hexdump -C /home/cactus/Descargas/arpeggio.gp  | head -4                                                                                                                                                                    
00000000  50 4b 03 04 14 00 00 00  00 00 00 00 00 00 00 00  |PK..............|
00000010  00 00 00 00 00 00 00 00  00 00 08 00 00 00 43 6f  |..............Co|
00000020  6e 74 65 6e 74 2f 50 4b  03 04 14 00 08 00 08 00  |ntent/PK........|
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

The pattern would look like this: \x40\x4b\x03\x04. This did not work. But I change the priority value to 100 an this finally works. The final code looks like this:

<mime-type type="audio/x-gtp">
    <magic priority="100">
        <match value="\x50\x4b\x03\x04" type="string" offset="0"  />
    </magic>
    <comment>GuitarPro file</comment>
    <comment xml:lang="es">Archivo de GuitarPro</comment>
    <glob pattern="*.gp"/>
</mime-type>

And it's finally works!