An Internet standard that allows for adding media attachments to an email is called:

MIME, Multipurpose Internet Mail Extensions, is the standard that lets email carry media attachments.

DHCP automatically assigns IP addresses to devices on a network, unrelated to attachments.

HDMI is a cable interface for carrying audio and video between devices, not an email standard.

FTP transfers whole files between a client and server but is not the email attachment mechanism.

MIME encodes non-text content like images and audio so plain-text mail systems can carry them.

It also labels each part with a content type such as image/jpeg or text/html.