As I have currently a bit of trouble with Dolphin renaming my files willy-nilly, I though that: Currently Dolpin displays an icon depending on the file suffix, e.g. one "image" icon for ".png", and another for ".pdf" - irrespective of the true file type (which is ontained by a call to the functionality of 'file' I suppose), which can be checked in the "properties" dialog. Showing a file icon depending on the actual file type instead of the file suffix would be the Right Thing to do. It would alert the user to a type problem if a ".png" is displayed as a pdf for example. If there is no suffix in the first place, the icon displayed depends on the file type in any case. So one would also be consistent here.
That is KFileItem in KIO iirc and is done for speed.
I suspect a related issue (possibly the same) is that in its Open dialogue Okular only shows as "All Supported Files" those with a .pdf suffix, which is confusing if PDFs are saved without it. However, if "All Files" is selected the correct icon is shown.
Forget that. Dolphin shows the correct icon so this is not the same bug, if the bug actually still exists.