It has been 7 days since the last response to the thread and I received no further complaints, so I've put the criterion into effect:

I've done slight tweaks compared to the original proposal:
* Help viewer has been included in the covered application types.
* Primary/default application is the one that must satisfy requirements in case of multiple same-type applications present (per the proposal by KDE folks mentioned in this thread).
* "only one of said applications must satisfy the requirements" has been changed to "at least one of said applications...", which has the same meaning but is clearer (I hope)
* "file browser" was renamed to "file manager", which seems to be the more official term (when looking at wikipedia)
* application type examples were put into a footnote, to make the criterion itself more readable (and avoid some potential confusion when people see exact application names in there and miss the "e.g.")

I'll work on adjusting associated test cases, to put them in line with the new criterion.