I have a bunch of files that have .png extension. However, some of them are actually gif files. Any of the files, I can open them w/ gimp or view them in firefox, but when I try to open them with the Eye of Gnome viewer, it gives an error "Could not load image: Fatal error reading PNG image file: Not a PNG file"

Is there:

1. Another image viewer that can open files with incorrect extensions that I could use as default? It would need to have the ability to zoom in and out on images, and scroll through all the images in a folder via arrow keys.
2. An automated way to rename the files that are actually gif files to the correct extensions?

Choice 2 is preferred, but I don't now how to tell what type they are. I would be able to put together a bash script to do the renaming if there is a command that could tell me what type they are.