I forgot to say that I downloaded to a Mac using Safari and I am using Illustrator CS4. On second look, it appears that all of the Illustrator files I try to download from Exchange have the same problem...they are invisible.
Okay, sorry for the wasted bandwidth. After further research, these files appear to be symbol libraries. I need to put them into the proper folder. That wasn't clear from the description, but I should have figured it out.