That regkey controls what types of files are "skipped" when the conversion process attempts to turn them in to html/text for indexing. In effect, what happens is that the email gets indexed (turned in to html), and then the attachment is archived "as is" (as in not turned to in to text/html...which would be messy for binary files). The metadata concerning the attachment _is_ recorded and indexed (so you can search on name, etc), but the contents are not.
So for example, if you had "shortsellthisstock.gif", in the gif was the typed words, "ABCX!", and you sent that to your accountant. If EV archives that mail message, both the message and the gif are archived. You can search on the message metadata, or the gif's file metadata, however, you couldn't search for the "ABCX" that's contained in the gif itself.
Does that make more sense?
micah