When I installed Slackware 64 I fired up gimp I could only open/save the native XCF files. I googled for a while with no luck. I downloaded the latest stable source (and required dependencies) and built with src2pkg (this lets you build multi-core support BTW). I installed the new pkg and still had the same problem. More googling still turned up nothing. I turned to the #gimp channel on irc, where they were convinced that src2pkg was a useless script and I should just build it manually. I knew that there had to be something simple I was missing.
The plugins that handle opening / saving other files types are plugins. Slackware installs the defualt plugins to
This is not where GIMP is looking for them. You have to tell GIMP to look there by adding that location to the Edit->Preferences->Folders->Plugins page.
Modules, Interpreters and Environment seem to handle this on their own, but the plug-ins do not.
Hope this helps someone.