Display "executable" in info panel instead of "unknown"

Summary: Executable files often have no extension, so MIME detection doesn't work. We can make an educated guess that octet-stream files with execute bit set are executables.

Reviewers: #ark, elvisangelaccio

Subscribers: kde-utils-devel, #ark

Tags: #ark

Differential Revision: https://phabricator.kde.org/D24383