I'm unable to get GIMP to recognize any image file format other than its own .XCF format. Any way I try to open an image file (file open dialog, drag and drop, set GIMP to be default editor, etc.), GIMP simply responds with an "Unknown file type" error. Image files other than .XCF don't appear in the file open dialog unless the "All Files" option is selected, and even then GIMP can't generate thumbnail previews of them.
I've checked the preferences to make sure GIMP is pointed at the correct plug-ins directory (D:\\PortableApps\PortableApps\GIMPPortable\App\gimp\lib\gimp\2.0\plug-ins, in my case) for the file format translators, but everything looks fine. Am I missing something?