I Need Some Advice

Let's say a user comes up to you and asks why her attachment was denied. Let's say the attachment wasn't denied whatsoever. At some point, the user understands that not having a file is different from having a file and not knowing what to do with it.

Then, just as a theoretical exercise, say the user prompts you to explain why an attachment would be denied in the first place. Now you know for a fact that even saying the phrase "MIME types" would cause the user to get a headache and then start bitching that you're deliberately trying to talk over her head.

How do you teach your user without making it so you both want to put a bullet in your brains?