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/vnd.comicbook-rar
), so 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.