What and where is the cid notation mentioned in the docs ?

Reference this URL:

https://www.soapui.org/soap-and-wsdl/headers-and-attachments.html#2-Attachments-and-Inline-Files

Adding Headers and Attachments | SOAP and WSDL

Quote:

"Adding Headers and Attachments | SOAP and WSDL We’ve attached a file from our file-system and refer to it using the cid: notation, which sets its type to “CONTENT”."

Problem:  the url link to the cid in that sentence is broken.

https://www.soapui.org/GenericError.htm?aspxerrorpath=/cid:<attachment%20name>

It looks like the cid notation might be exactly what I am looking for, but since the link is broken, I am stuck.

Can anybody help with this ?

Thanks,

Charles R.