A CDE Definition
A computer file that rides along with an email message. Attachments are widely used to transfer photos and documents to another party. However, very large files and executable files are generally prohibited, which is why users sign up with a file sharing service that has no restrictions (see file transfer service and cloud storage).
Whether using a computer, phone or tablet, the process is the same. Users compose a message to a recipient, select Attach from a menu and then browse a folder hierarchy to find the file or files to attach. One drawback to attachments is that the attached file grows about 30% larger than the original. It must be encoded into a text-only format in order to pass through all the servers in the Internet mail system (see 7-bit ASCII).
Attachments Can Be Dangerous
If the receiving mail server accepts an executable attachment (most do not), and the email recipient clicks it, the program will be run in the user's computer. This is a common method for implanting a virus or Trojan (see dangerous extensions). See email and how to transfer a file over the Internet.
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