GRUB module files get detected as audio/x-mod

GRUB module files (e.g. /boot/grub/x86_efi/*.mod) get misdetected as mimetype audio/x-mod, since this is the only mimetype associated with the .mod extension by shared-mime-info.

GRUB module files seem to be standard ELF relocatables, so it should be possible to disambiguate based on magic detection.