Tracker doesn't know anything about mime-type aliases
Tracker seems to treat mime-types as "strings" and checks them for equality. So it means that it doesn't know about mime-type aliases, which are 100% equivalent but non-canonical mime-types.
For example, CBR comic books, before they had an official mime-type from the IANA, used the
application/x-cbr mime-type. The IANA provided an official mime-type (
application/x-cbr became an alias. gnome-books, to list those books, queried for files with
application/x-cbr mime-type. The results should have included
application/vnd.comicbook-rar files but did not.