From my experience if there is anything wrong in the image, doesn't matter whatever extension it is, WWF define the uploaded image is of a wrong file type.
Even with WWF v9.04 I had this message while uploading a picture of Fleece Jacket for a Factory profile. The image had the extension .JPG but it was not JPG or GIF or PNG. to say exactly it was bad formatted JPG. So, WWF generated the error.
So, from my experience, it is not or never a bug. But I am sure boRg will define what it is more perfectly.
-XIV- you can use a graphics converter to convert your image to JPG and then upload it. Hopefully, you won't see any error message. Do not use Photoshop/Firefox. It's an editor, not a converter. I didn't have proper results from them.
I use 'IrfanView' to convert images.
www.irfanview.com - it's free and tiny.