[herokusaki]

You may be right about the extensions.

Thanks for suggesting gvfs. I'll investigate it and similar databases from other packages (I know at least KDE has its own).

[e12e]

I suppose this might be too late, but it might be worth having a look a tracker[1], and tracker-search[2]. Alternatives include recoll and Beagle (now defunct?).

[1] https://wiki.gnome.org/Projects/Tracker

[2] http://www.mankier.com/1/tracker-search