attachment

Attachment is any file, uploaded to post, page or added to email. It can be an image, text file, video or audio media file, etc. Attachments might be harmful to your server or local computer because of malicious scripts they may contain, so it is recommended that you do not open such files in case you’re not aware of original message sender/page author. Target files at downloadable links are also considered as attachments.

For better explanation, you may check the following link to learn how to attach file to your message in WordPress templates by using Contact form 7 plugin.