Showimage should set MIME and thumbnail

[humdinger]

This is hrev56522.

If you have a folder with images that don't have a MIME type yet (maybe downloaded with wget), ShowImage should set the MIME type when opening it and trigger thumbnail generation.

Double-clicking a generic file does that already, but that's Tracker's doing, I suppose.
If you continue to browse through the files in that folder, ShowImage already fills the width/height attributes.