Upon some investigation, it seems that `xdg-mime` is returning a wrong
mimetype for image files inside sandboxed shell. However, it returns a
valid mimetype outside tracker sandboxed shell. Here is the result of
running `tracker-extract` on one of the image files :
http://fpaste.org/169097/21161511/ from which it appears that it GIO
can't recognise the mimetype of the file.
Can you please have a look and show some pointers where I could have messed up ?