“They should be properly labeled, so people know what they contain.” What is this sentence about?

OK, if you clicked “medicine bottles,” why would you do that? This is a micro-course about emails, remember? Imagine this situation: you’re having a difficult conversation with a client, and then you need to check an important document you received via email. When you open the email, you find out there are seven other files attached to the message, so you have to open all of them to see which is the one you need. How annoying is that? Remember to label your email attachments properly, so people know what they contain.