Although since KDE 4.10.0 Gwenview knows to open images with incorrect extensions, Dolphin still won't generate thumbnails for them when Preview is enabled. For example, if a jpg file has the extension png, no thumbnail is shown and instead we see the default image icon.

Reproducible: Always

Steps to Reproduce:
1. In Dolphin, browse a folder which contains an image.
2. Make sure the image's extension is incorrect. If it's a jpg make it a png or the other way around.
3. Click the "Preview" button to enable thumbnails.
Actual Results:  
You'll see the default image icon.

Expected Results:  
You should see a preview of the image.

This was a known issue in Gwenview but since KDE 4.10.0 it was fixed there. I thought this would also fix thumbnails automatically but it seems not, so I believe it needs its own attention.

Although no one should use incorrect extensions, this happens shockingly often for images I get online. For that reason a lot of stuff I save from galleries doesn't preview, which is why this bug can frequently be a problem.