We can't use string equality to find matching mime-types, as this
doesn't handle mime-type aliases. For example, the application/x-cbr
mime-type became an alias to the IANA registered
application/vnd.comicbook-rar mime-type.

We need to check mime-type equivalence using
g_content_type_is_mime_type() instead of a string comparison to handle
versions of shared-mime-info where application/x-cbr is an alias.

Closes: #972