Comment 6 for bug 206892

anyfile.desktop, anyfile.theme should always be a valid Desktop Entry file. An empty file is not a valid Desktop Entry file, which is why you're seeing that dialog.

I have no problems editing a .config, or .index file here, they're empty text files and open in GEdit as I would expect. An empty desktop entry file also does more or less what I'd expect (an error dialog, though a more helpful error message such as "This is not a valid desktop entry file" would be prettier.) It would be nice if we gave an "Open With" entry for those files as well though, so that users who know what they're doing can edit them by hand more readily.