I'm by no means a Linux expert, so you should proceed with caution with the following, by backing up your current /usr/share/file/magic* files (or wherever they reside in your system).
The following is an answer I found at StackOverflow (a general purpose technology forum):
You can man magic for a description for how to create your own magic file. Then use file -C -m <your magic file> to compile it, and file -m <your magic file> to use it.
From what I skimmed on the web, it seems you need to recompile the magic.mime file so the changes take effect.
You may also have to restart your Apache server so it takes the changes (?).
Again, I'm no expert at this level and have currently no one to ask about this that knows more than I do (except the PKP Team).
Ramón Martins Sodoma da Fonseca
Analista em C&T
Coordenação de Tecnologias de Informação
Instituto Brasileiro de Informação em Ciência e Tecnologia - IBICT
Ministério da Ciência e Tecnologia - MCT