Files sent by email. Attachments are commonly photos or documents.
Beware when clicking on an attachment. They are one of the most common ways viruses are spread. It’s good to know that attachments can only infect your device if you click on it or try to open it. So, don’t open attachments if you are not 100% sure of the sender.
When sending attachments to friends, consider sending a separate email that gives them a heads up it is actually you sending the attachment.