You can get the behaviour you want by selecting "Determine image type from content (slower)" in gthumb preferences. The default is for this not to be checked, as it would be a lot slower showing directories with large numbers of images.
You can get the behaviour you want by selecting "Determine image type from content (slower)" in gthumb preferences. The default is for this not to be checked, as it would be a lot slower showing directories with large numbers of images.