What does a blank file type mean to the ASM file type allowed list?

If you have a wildcard file type allowed, then the policy should let all file types through except the ones listed in the disallowed list. So, the blank file type shouldn't really matter as long as you have a wildcard listed. That said, I've never seen a blank file type listed in a security policy. You should be able to delete the blank file type with no repercussions to your policy. Ideally, you should allow only the file types that your application uses, and then take out the wildcard...this really tightens things up for the security of your app. The wildcard should only be used while you are learning the file types that your app uses.