"Missing Plug-in" in Mail

Lately, in Apple Mail, instead of being able to see images attached to emails, I just get a message stating, "Missing Plug-in." I'm not sure if this is a related problem or not, but also, when clicking on an image and hitting the spacebar to bring up a preview, nothing appears there either.

After searching the web for help, I've tried restoring defaults in the Java Preferences application, but no go. I've tried downloading and installing Java off the apple website (http://support.apple.com/kb/DL1515?viewlocale=en_US&locale=en_US) but that didn't work either.

Any suggestions on how to fix this annoying issue?