When using ImageMagick's convert tool, the output defaults to the input format if no file extension or format is specified.
The temp file currently has no extension, so a $config['thumb_ext'] value has no effect on the image.
By appending the thumb_ext to the temp output file, it will convert the image to the intended format.
You can see this issue present on lainchan, where thumbnails have a .png filename but are not really PNG files when the input is a .jpg, for example.
